Auto merge of #2679 - sbeckeriv:alias, r=alexcrichton

Auto merge of #2679 - sbeckeriv:alias, r=alexcrichton

Command alias or Alias command #1091

Dearest Reviewer,

This pull request closes #1091 which is a request to support aliases.
This is not as powerful as something like git's alias, however, I think
it sticks true to the original request.

I high jack the processing of the args. After a few flags are checked
and the args are parsed I check the config file for alias.COMMAND. If it
is there I split it like args does and replace args[1] (the original
command) with the alias command and its 'flags'.

As an extra measure I output the alias command with warn. I would be
willing to drop that or put it behind a verbose flag. Also the docs have
been updated.

Thanks!
Becker

<img width="784" alt="screen shot 2016-05-12 at 10 23 59 am" src="https://cloud.githubusercontent.com/assets/12170/15226012/d18a3336-1835-11e6-94c9-875a63a79856.png">