Add mark all as read command

Fix compilation with d:internalRenderer

When I rebased my toggleread branch onto master, some conflicts didn't
get resolved correctly.

Allow toggling read status from post selection

This commit changes many of the main types to be ref objects. This
allows their state to be updated from afar. It was necessary here to
let a post be altered from within a termask promptList and have the
changes immediately shown in the list of choices.

Fix crash when displaying very long link in post

Use post link as GUID when none is provided (closes #8)

Fix typo in default config

Use more specific 'CatchableError' instead of 'Exception'

Load feeds in parallel at the start

This (finally!) closes #11. Now that we explicitly tell the compiler
that html2text.handle is GC-safe, we can call it from a parallel block,
which allows us to parallelize the feed parsing. It turns out that it's
actually only a small time improvement, but it's still nice.

Only require the nimpy import when missing -d:internalRenderer

Use published version of patched FeedNim

Show helpful message when managing no subscriptions

Escape fields in subscriptions file

This fixes #20. Previously, feed titles that had a comma in them would
not work, but now, they can be added without issue.

Add option for using nim-html2text

Use constructor to create Subscriptions

Constructors are preferred over simply creating an object of the right
type to ensure that any fields that are added to the object in the
future are updated throughout the codebase.

Add support for editing subscription title

This commit adds a new dependency on the noise library, a Nim
implementation of linenoise. This is being used instead of rdstdin in
the Nim standard library (which wraps linenoise) because it's nice to
give the user the current name as a default when editing, which rdstdin
doesn't allow.

Use generic version of promptList for choosing feed

Implement manage subcommand with unsubscribe support

Remove accidentally included print

Use cmpIgnoreCase for sorting subscriptions

Using the normal cmp function caused feed with capital letters in their
titles to be sorted above feeds with lowercase letters. It's more
intuitive to have the feed names sorted in a case-insensitive manner.

Add link to nim-html2text