Enable spell checking for more elements

Spell checking is only active on the "base text" (not inside directives). This is unfortunate as many directives contain normal text which would profit from spell checking, e.g. .. note:: directive and similar.

I advocate for enabling spell checking for as much elements as possible and only deactivate it for sphinx which are (almost) certain not to contain content worth spell checking, e.g. literal blocks. I rather err on the site of spellchecking more than needed, as a wrongly marked typo can be easily ignored, whereas the contrary finding a typo which is not marked is very hard. Therefore, most elements should be checked.

Pull request #71 is one step in that direction.