Adds a set of words that I consider "known" (but that should be in the
most frequent word list anyways). This set can be populated by
invoking `mark-last-russian-word-as-known` after display, and is
automatically persisted.
Right now there's nothing automatically loading it back in, just as
there is nothing loading any of this automatically, that's for the
future.
Change-Id: I51ee4f37114c6b95925e8ad5bdc5dc9b8657bdad