bootstrap3 and alternate syntax for specifying profile

I've added a profile for bootstrap3 grid and a bit of code to handle conf["profile_name"].

Reason for the "profile_name" key is this. In Pelican, Markdown extensions are specified like this:

    MD_EXTENSIONS  = ['codehilite(css_class=highlight)','extra']

With this syntax I could not easily specify grid profile to use. With "profile_name" key, I can do:

    MD_EXTENSIONS = ["grid(profile_name=bootstrap3)"]