Added support for .mpd (and others)

[piersoh]

It would be useful to add support for media presentation description (MPD) files (.mpd) which are basically XML files. Here's the output from mdls:

kMDItemContentType     = "dyn.ah62d4rv4ge8046de"
kMDItemContentTypeTree = (
    "public.item",
    "dyn.ah62d4rv4ge8046de",
    "public.data"
)

Though when I drop an MPD file on the Inquiry File dialogue in the app it suggests an additional UTI is associated with it:
dyn.age8046de

Also It would be nice to have a way - even via the odd command line operations - to add arbitrary files types a bit like is possible for qlstephen by editing its Info.plist file.

[sbarex]

Implemented on release 2.2.20.

For manual support other formats see the README.md.