This Pull Request fixes/closes #2523 & closes #2308.

Custom syntax highlighting themes can be loaded from a .tmTheme file.

If syntax in theme.ron (added in #2532) is a filename that exists in the gitui config directory, it's used as the syntax theme. If not, gitui falls back to selecting from the default theme set.

For example, if you put the following in theme.ron:

(
    syntax: Some("mytheme"),
)

gitui will load ~/.config/gitui/mytheme.tmTheme.

I followed the checklist:

• I added unittests
• I ran make check without errors
  make check reports a duplicate dependency error:

  error[duplicate]: found 2 duplicate entries for crate 'base64'
  

• I tested the overall application
• I added an appropriate item to the changelog