Add possibility to make mapping type explicit

[ericcornelissen]

Currently, the mapping file is parsed base on the file extension. This could be extended by allowing the user to explicitly mention the format used in the file (if e.g. a different file extension is desired).

One option is to extend the current [-m] [--map] options:

# Comma
$ wordrow -m example,md

# Semicolon
$ wordrow -m example;csv

# Colon
$ wordrow -m example:md

Another is to introduce a new option, which could be confusing w.r.t. the ordering of arguments, e.g.:

# One mapping
$ wordrow -m example -t md

# Two mappings (both would be equal)
$ wordrow -m foo -t md -m bar -t csv
$ wordrow -m foo -m bar -t md -t csv