feat: allow checks to be disabled

Closes #1
docs: explain each check and how to disable them
docs: link to article on passive voice

Adds a link to an article by G. Pullum (author of Cambridge Grammar of English)
explaining why the passive isn't actually a problem in technical writing most
of the time, and isn't worth correcting. A good read.
0.4.0
docs: fix example output
feat: add annotation API and shell script
fix: do not detect lexical illusions for non-words
chore: update dependency versions
chore: fix typo in tests
docs: add travis badge
fix: do not detect 'so' in the middle of a sentence
0.5.0
docs: add CLI
feat: add flags to CLI for different checks
docs: explain CLI options
fix: don't complain about words starting with "so"

:rocket:
docs: add app section
docs: note about apps
docs: add nitpicker tool

Closes #4
fix: handle empty strings