Support for POD formatting (e.g. highlight bold as bold) and spell-check enhancements

Full details in the commit (6b4ebd06de5d90cef43cd40aff28ed2a5d0886de), but for ease of reference:

* Adds support for highlighting POD inline text, for example bold as bold and so on.
* Adds support for spellchecking POD headings (=head1 etc.)
* Adds support for spellchecking bold and italic inline text.

These changes are generally very useful for identifying typos and incorrect formatting during peer reviews, also simply make editing easier.

Please let me know if you have any questions or comments.