Support for Texy markup language

[OndraZizka]

There is a PR already, #485
but I am creating this so it has a proper issue.

Let's add a support for Texy markup. Texy is written in PHP but can be accessed as a (Docker) service.
It would be up to the user to set up the service before runnign JBake IF and and only if they want to process .texy files.

[jonbullock]

I'll schedule this for 2.7.0 for the time being, can add it along with Pebble (#246).