Improve error messaging

Add BIDI support

* add dir=auto to title and articles

This enables languages like Persian, Arabic and Hebrew to have proper
direction by "dir" attribute:
          https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/dir

Improve licensing details

This is kind of based on the REUSE specification, but I find the spec
annoying in some respects so this is not entirely compatible with their
advice. I will pitch these alterations upstream at some point.

Check for empty feed title

For feeds with no title, openring places a link with no clickable
text on the byline ("via ..."). Instead, we replace the empty title
with a sensible substitute (URL.Host).

Additionally, fix an unhandled error.

Check for error value in url.Parse

I noticed I get a panic when running openring with http://antirez.com/rss
The panic happened because the url.Parse returned error is not assigned
to a variable.

    <antirez>
    panic: runtime error: invalid memory address or nil pointer dereference
    [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1119fd6]

    goroutine 1 [running]:
    net/url.(*URL).ResolveReference(0xc0004c0bd0, 0x0, 0x0)
            /usr/local/Cellar/go/1.16.5/libexec/src/net/url/url.go:1075 +0x36
    main.main()
            /Users/gianarb/git/openring/openring.go:152 +0x9c

There is another problem, the URL is not valid for the url.Parse
function for some reason. But this is an issue for another day.

Signed-off-by: Gianluca Arbezzano <ciao@gianarb.it>

Add -S flag for specifying a file with feed URLs

Remove link to pre-compiled binary

allow custom date formats

This adds a new datef function instead of changing date function in an
effort to keep backwards compatibility.

datef is intentionally used in the example to make it clear to the user
that it is possible to set a custom date format.

Add support for resolving relative feed URLs

I think this is probably more like a bug in github.com/SlyMarbo/rss
but at least this fixes it for now

Update README.md

Rewrote flag parsing to use flag-like interface

style: keep articles from being horizontally compressed

In the default style for openring articles are arranged
horizontally.  This is fine with a small number of articles
or a wide screen, but can look squashed when you don't have
those.  Current behavior, on a "Galaxy S5" in Chrome Devtools:

    https://www.jefftk.com/openring-example-nowrap.png

Instead, give each article a minimum width and tell flexbox to
wrap as needed.  To maintain spacing between the articles
without adding spacing around the edges, follow the approach
described in:

    https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Mastering_Wrapping_of_Flex_Items#Creating_gutters_between_items

This gives us new wrapping behavior on narrow screens, without
changing behavior on wider screens:

    https://www.jefftk.com/openring-example-wrap.png

improve handling of feeds with no summary

When testing Openring with Atom feeds like http://blog.davidchudzicki.com/feeds/posts/default and http://www.givinggladly.com/feeds/posts/default I noticed it wasn't able to pull out a text snippet.  Falling back to Content if Summary is null fixes this.  Content is html encoded, however, which means that we also need an UnescapeString or else Sanitize won't remove the HTML tags.

Add flag to specify number of articles per source

Fix panic when too few feeds are retrieved

    panic: runtime error: slice bounds out of range

    goroutine 1 [running]:
    main.main()
    	/home/simon/src/openring/openring.go:121 +0xef4
    exit status 2

Update README.md

Initial commit