Add public API option 'sumStartsWithNEquals'

Docs, code, unit tests, visual tests.

Resolves #304
Fix incorrect usage of `sumStartsWithNEquals` option

https://github.com/mathquill/mathquill/pull/338#discussion_r18552362
Merge pull request #338 from mathquill/feature.sumStartsWithNEquals

Add public API option 'sumStartsWithNEquals'