Add TWBT as a printmode option

[Pidgeot]

TwbT is now activated by setting [PRINTMODE:TWBT], so it would be good to have this as a printmode option. If TwbT is not available vanilla DF defaults to STANDARD, so it won't break anything.

[Issue created by PeridexisErrant: 2014-09-26]
[Last updated on bitbucket: 2014-10-11]

[Comment created by Pidgeot: 2014-10-11]
Massive refactoring work
Allow more flexible placement relative to DF directory
Add TWBT detection and print mode options (fixes #31)

→ <<cset 4daf6a30f367>>

[Comment created by Pidgeot: 2014-09-26]
I already have local changes ready to add this (and TWBT_LEGACY) as options if TWBT is found in the df/hack/plugins folder, but as far as I could tell from the TwbT forum thread, this was something that mifki might change fairly soon. Because of that, I won't be committing this until I make the next build.