Added doubleend theme from Nettuts+

This commits have been squashed:
- Added doubleend theme from Nettuts+
- Better git prompt on doubleend theme
- Syntax fixes, precmd inside $PROMPT to work with Ctrl+L
- Refactoring doubleend theme
- Cleanup, comments added
- Variables wrapped in double quotes in spacing()
- Modified $PROMPT to single line; cleanup.
- Modified CTRL+L behaviour to execute precmd() prompt line
- Renaming functions. Fixes git autocompletion
- Theme refactor. Works on CTRL+L and resizing
- Simplification of if-else
Add return codes to agnoster theme

This adds the ability to show the return code of every process in a pipe chain,
as long as at least one of them returns a value other than 0.
There is the option to disable this behaviour as well.
Merged from robbyrussell/oh-my-zsh/pull/2573 (by @diogovk)
Merge remote-tracking branch 'LFDM/cleaning_lib' into dev

Conflicts:
	lib/history.zsh
Merge remote-tracking branches 'ncanceill/plugin-git' and 'ncanceill/easymerge' into dev
Merge branch 'omz-bootstrap' into dev