Use Mukti as a fallback font for Bangali

It fits sylistically with DejaVu and has both a normal and bold version
Support more recent unifont versions

Unifont's name changed from "unifont" to "Unifont" at some point,
probably with 6.0.

This commit causes the more recent caps to be preferred, but will
support either version.

Closes gravitystorm/openstreetmap-carto#429
Use Gargi for Devanagari rendering

Gargi was considered the clearest of the available fonts
Add TSCu_Paranar as a font for Tamil

There aren't really any other matching options for Tamil fonts
Add Tibetan Machine Uni to fonts list

There aren't really any other Tibetan options for fonts
Add Mallige for Kannada

Mallige has bold and italic versions.

Kedage is another option for a Kannada font, but it seems to be impacted
by mapnik/mapnik#2195
Use the correct name for Tibetan Machine Uni
Add documentation on fonts and package names for #414
Merge pull request #414 from pnorman/s_asia

Add South Asia fonts