bash: remove rvm stuff from prompt
bash: add prompt-related functions from epeli

Nice functions, next up is refactoring all the prompt stuff into
prompt_command.
bash: remove extra prompt stuff from bashrc

Setting xterm title was defined in two places, remove the other one.
bash: refactor prompt setting into PROMPT_COMMAND

Cleaner this way, again inspired by
https://github.com/epeli/vimconfig/blob/master/others/bashrc. Tweaked
the patterns to work even with my setup (maybe colored git messes them
up? dunno).