mutt: Remove failed HTML email experiment.

I was trying to use the HTML view by default in emails from Trello, because
their plain text emails are so bad. It didn't work out.
git: Update difftool preferences.

I'm mostly using vim in the terminal these days.
vim: Use filetype-based indentation.
vim: Add handlebars support.
vim: Stop remapping arrow keys.

I'm no longer tempted to use them, so all this does is trip up people I pair
with.
vim: Leader commands to change indentation.

Sometimes, a project uses different indentation settings to my defaults.
vim/bash: Set up golang support.