Spell checker support for Markdown files

This patch adds a binding for the NetBeans Spellchecker API. It is rather basic and treats the source document as plain text, i.e. it scans the document for words (contiguous sequences of letters) and does not use information from the AST to exclude parts which most probably do not contain natural language (names of embedded HTML tags, URLs in link elements).

The display of detected spelling errors can be disabled under Options > Editor > Spell Checker.