This changes a few things:
- All syntaxes and themes are now stored (as submodules) under
assets/syntaxes and assets/themes
- The default directories for syntaxes and themes are "syntaxes"
and "themes" (used to be "syntax" and "themes")
- The "bat cache" command can now take a `--source <dir>` and
`--target <dir>` option.
- The cached files have been renamed to "themes.bin" and "syntaxes.bin"
