Vim: rename pascalCase snippet helper to pascal_case_basename

I am about to add another function here and realized that the name I had
originally given to this one wasn't as good as I like it to be. Also, I
realized that the naming convention in Python is snake_case, not
camelCase, so I made that adjustment at the same time.
Vim: add BEM css snippet

We are using the BEM methodology and breaking the styles for our
components down into separate files that match the BEM block name. I can
leverage the power of UltiSnips here to make my work more efficient.