Custom trestle width

[monkeywithacupcake]

adds new variable: leftsize to trestle skeleton and html to allow for custom view split on medium and larger screens
closes #36
closes #37

[monkeywithacupcake]

The default for leftsize is 5 whether or not the user includes the yaml leftsize because the html template checks
If the screen is smaller than md, it stacks.
Else, the left side splits using the leftsize variable, which can be between 0 (invisible) and 12 (the whole width)

Here is an example with leftsize: 3

Here is an example with leftsize: 8