Convenience functions for simple workflows #288

Having started to use this package coming from quanteda in R, I feel some convenience features could greatly streamline common workflows:

• Top features in corpus/subset of corups/document
• Filtering DTMs by eg. proportion of documents that contain a word, or a regex pattern
• Clearer documentation around DocumentTermMatrix vs dtm() functions, eg. on the ordering of terms
• Coding a document term matrix with e.g. a sentiment dictionary (cf. quanteda.dictionaries) package inR

I'd be happy to contribute PRs for these, if any/all are desired functionality. That said, I'm quite new to the ecosystem so if I'm missing something do let me know!