read themes from bat's cache

[data-pup]

This does not entirely address #19, but does address this comment.

This pulls in some more code from bat to read themes from ~/.cache/bat/. Tested locally on my machine (with an existing bat install) and this does work properly. That said, I know this does not include tests: I would be happy to add some, just unsure of where those would go.

[dandavison]

@data-pup Thanks very much for this! It looks great; very clean. I've tried it out, rebuilding the bat cache with bat cache --build and can see that delta is picking up new theme and syntax files. I guess I have no excuse not to work on an emacs-lisp sublime syntax definition now...

So the one blocker is we need to figure out what's up with the Windows build: https://travis-ci.com/dandavison/delta/jobs/269918710

If you would like to add some tests, src/delta.rs would be a reasonable place. (It already has some tests related to theme).

[data-pup]

@dandavison Hmmm, so I tried running cargo build --target x86_64-pc-windows-msvc on master at 13e39b1de9ef0a8324b882fade04eeff65241f40, and 1c3e7baf0f96bc0ff07da8d0aab9b1f2d15b02d2, and ended up seeing the same build errors. I am suspicious that something elsewhere may have broken the build? I'm not quite sure how to debug this tbh 😖

[dandavison]

Just FYI I'm going to be offline for the next 10 days so will go quiet, but I'm keen to get this PR merged. Thanks for looking at the build failure.

[dandavison]

You're right, I see now that Windows builds have started to fail on master also, so unrelated to this PR. Thanks very much then! I'll merge this now.