better installation instruction

As requested in https://todo.sr.ht/~nhanb/mcross/3#event-57035
deprecate Windows support
fix preformatted block with alt text

I really need to do proper parser tests one of these days...
shoutout to picross
unified conf file / cli args

Config by priority: default > conf file > cli arg

It supports long form and short form but quite limited:

- Key-val style only, no on/off switch style e.g. `mcross --dark`
- Isn't POSIX-compliant i.e. `-d1` doesn't work

The Click library might help but I'm not sure if strict POSIX compliance
is really worth complicating the dep tree...
fix empty body/charset case; use regex
parse and correctly use mimetype+charset from meta
correctly render text/* mime type
fix relative link redirects, bump 0.5.14

dark mode with --dark cli arg
bump 0.5.13, add mailing list instructions
Parse absolute url links with scheme omitted

This enables McRoss to parse links that start with "//", which is just
"gemini://" but with "gemini:" omitted but implied, in accordance with
the Gemini spec, section 1.2.

Additionally, this patch takes McRoss all the way to #0008 of the
conman.org torture test, instead of #0002.
move CLI parsing out of controller, bump 0.5.12
Add CLI arguments to pick custom fonts

Users can now pick custom fonts via args `--textfont` and `--monofont`.
This is a WIP patch. Todos:
- Add more font candidates to the "default" font lists
- Add help text for arguments
- Perhaps a config file
show request time status, add request timeout
implement alt-shortcuts for buttons
fix bug when visiting relative url from root

enforce max header length too