This introduces compatibility with other tagged template plugins, like

babel-plugin-dedent, which is useful for constructing strings that

are further processed through some other means.
When a tagged template is encountered as a string, it will be preserved

(well, rebuilt) and left for Babel to later process. The rebuilt tagged

template will have its contents replaced with a normalized version of

the string that would normally go into the gettext function. That means

that if the tagged template were to reference other variables or

expressions (using ${...} syntax), those would be replaced in the

rebuilt one with the gettext interpolated %(...)s markers.
In order to make any of this work, we had to change how we prevented

further processing of nodes. We had previously walled off any resulting

nodes (which may be a gettext-like call or an interpolate call) from

further processing, so that we wouldn't end up with an infinite loop of

processing these functions, and we skipped children. Now, to allow Babel

to reach down into the strings, we now mark any resulting interpolate

or gettext-like function as having been transformed, and we simply

don't re-transform those nodes, but don't skip processing of children

either.
Unit tests and documentation were updated for the new support.

Unit tests passed.
Manually tested with some code in Review Board.
Viewed the docs and checked the generated output.