Add commandline option for theme

[jubalh]

If you run multiple instances of profanity with multiple accounts you might want more visual feedback of which instance is which.

So if you connect with profanity -a account1 you might want a different theme than with account2.

We need profanity -t theme to define the theme upon startup.

This is also interesting for @DebXWoody https://profanity-im.github.io/blog/post/how-to-run-profanity-instances-within-tmux/