Add support for tectonic typesetter

[Andriamanitra]

Adds support for using tectonic as the typesetter

closes #187

[Andriamanitra]

I pretty much just searched for "rubber" across the code base and copied what was implemented for the "rubber" type setter. It seems to work with the default settings with the default document already but any advice from someone who actually understands the code base would be much appreciated.

I didn't add anything to src/external.h or src/external.c as I'm unsure what they are used for and what the output should look like.