seqtk/tabtk: Update and move tabtk to separate build

The latest seqtk GitHub moves tabtk to its own repo, breaking HEAD
(chapmanb/bcbio-nextgen#592). This creates separate builds for both,
fixes the tabtk issue, and moves to GitHub versioning of seqtk/tabtk
since it doesn't have a formal release process.

Closes #1183.