Make label heading styles the default

[anselmbradford]

See background in https://github.com/cfpb/capital-framework/pull/632#discussion_r142753782

Current behavior

• The majority of the time label headings are used, not plain labels. This means a-label a-label__heading classes are generally needed.

Expected behavior

• a-label has the label heading styles and a separate a-label__inline class is added that negates the heading styles. a-label__heading is removed.

[jimmynotjim]

@anselmbradford should we add this to the v5 project? It'll require updating markup in a lot of places and probably shouldn't be considered backward compatible.

[anselmbradford]

Added it.

[jimmynotjim]

Scott and I were discussing this today as part of something unrelated and we had a realization, making the heading style the default would mean unsetting all those styles for the inline modifier. Currently, we're only adding styles, not removing them, so it makes sense from that point of view. One solution might be to make an a-inline-label atom just for inline labels. I'll leave it to you all to decide.