Improve Kinyarwanda stemming and form tags for verbs

[rowanseymour]

Instead of always stripping final vowels, generate --a and --e form tags, e.g. currently kugenda would get the form tags:

kugenda (kugend), -genda (gend), -genze (genz)

Should be:

kugenda (kugenda), -genda (genda), -gende (gende), -genze (genze)