Implements enough of the [terminal hyperlink specification][1] to allow
us to include links to PRs, where they exist, in the terminal output.

This specification is implemented by several terminals, including:
- kitty
- wezterm
- iTerm2
- Alacritty
- Windows Terminal
- Konsole
- xterm.js (VS Code's backing terminal implementation)

See [this tracking repository][2] for a full listing.

The ECMA-48 specification requires that terminals that do not understand
the OSC-8 command code silently ignore it; in compliant terminals that
do not implement the hyperlink specification, users will simply observe
no changes to the output of `stack-pr`. For cases where outputting these
escape codes is still undesirable I have added a `--no-hyperlinks` flag.

[1]: https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda
[2]: https://github.com/Alhadis/OSC8-Adoption/

Fixes #15.

GitHub lets you check if a PR is actually mergeable via the `gh` CLI by
retrieving the `mergeStateStatus` field. This field has a bunch of
values (see the [API documentation][1] linked below), but the one we care
about is `CLEAN`, indicating that the PR has no merge conflicts and has
passed PR checks.

I'd like to eventually add this information to `stack-pr view`, but it will
require some extra piping, and will slow down the command due to the
extra network request.

1: https://docs.github.com/en/graphql/reference/enums#mergestatestatus

delete_remote_branches formatted remote
branch refs with an extra single quote
around each branch. These would never
match the branch strings stored in
stack entries, so remote branches would
never get deleted. This commit removes
the extra single quotes.

For large or busy repositories, stack-pr commands can generate hundreds
of lines of output from Git subcommands being executed. Most of the time
these don't matter; the user doesn't care about them. This PR suppresses
them by default by redirecting stdout and stderr to
`subprocess.DEVNULL`. If this output is desired in a specific case, the
command-line flag `--verbose` (`-V`) can be used to avoid this
redirection.

The allows to have a local config to customize defaults for CL options.

Closes #22.

Closes #18.

stack-info: PR: #38, branch: ZolotukhinM/stack/3

GitHub deletes the remote branch automatically when the PR is merged, so
we don't to explicitly do that. Trying to delete it currently results in
an error like this:
```
error: unable to delete 'ZolotukhinM/stack/1': remote ref does not exist
error: failed to push some refs to '...'
```
The code for deleting the branch has been sitting there for a long time,
but until a fix in #27 it used incorrect branch names.

…ame_template (#40)