problem with config in command line  #341

If I run the follow command in current development version:

mkdocs serve --config=/home/lf/git/mywork/bb/bog/mkdocs.yml

it will raise error:

Config file 'mkdocs.yml' does not exist.

But if I run the same command use version 0.11.1

Everything is OK

Is there any thing wrong with code below in config

if 'config' in options:
    filename = options.pop('config')

Should it be:

if 'config' in options:
    filename = options.get('config')

Because when we run mkdocs serve , we will execute this block of code two times, filename will use the default mkdocs.yml in the second time, this file may not exist.