Improved .frow-container

[codysherman]

Firstly, is .frow-container the best name? It got the name from back when Frow and Bootstrap were most identical, and was to just differentiate between Bootstrap's `.container' and Frow's. Now it's mostly used to make whole pages not go too wide. Can we perhaps rename this, and provide more options. Sometimes it's too wide or too skinny. Frow's own documentation website found it too wide, but other projects have found it too skinny. Perhaps some type of "-lg" "-sm" system?