.bash_prompt: Avoid double-escaping ANSI codes

Fixes #399 and closes #402.
.bash_prompt: Add back escape sequences

Fixes #403.
.bash_prompt: Interpret escape sequences correctly in all cases

Closes #407.
Merge branch 'master' of github.com:mathiasbynens/dotfiles

* 'master' of github.com:mathiasbynens/dotfiles:
  .bash_prompt: Interpret escape sequences correctly in all cases
  .bash_prompt: Add back escape sequences
  .bash_prompt: Avoid double-escaping ANSI codes
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