docs: document feature analysis for Run.style
acpt: add run-char-style.feature

Acceptance tests for character style support, implemented as the
read/write Run.style property.
run: add Run.style getter
run: add Run.style setter
acpt: add par-add-run.feature

Retroactively added an acceptance test for Paragraph.add_run(). Made
some stylistic updates to naming in steps along the way.
run: add style param to Paragraph.add_run()
docs: add user documentation for Run.style
release: v0.5.2 #17 feature: character style