Initial support for insertables

Those little things like $1 and PLURAL syntax that is annoying
to type by hand, especially on tablets where copy and paste is
cumbersome.

Example suggester for MediaWiki with tests. Like checkers I
imagine most suggesters implemented next to the yaml configuration.

Bug: 38350
Change-Id: Ibf8d7a3ad7f80101f90fe936ebdd0d1af61797d2