Add web-search plugin.

This plugin adds google, bing and yahoo commands to launch the default
web browser to do web search:

e.g.

    google oh-my-zsh
    bing what is zsh
Fixed recursion. Git not needed for it to work.

Avoid infinite `cd` loops under certain conditions. Try getting `.venv` from the current directory (not necessarily always using git)
Merge pull request #1839 from Kronuz/patch-1

Fixed recursion. Git not needed for it to work.
Merge pull request #1837 from xuhdev/web-search

Add web-search plugin.
Merge remote-tracking branch 'robbyrussell/master'