Add keywords field to package metadata

[earlbread]

This patch adds the keywords field to package metadata. (#100)

I named keywords metadata to packageKeywords because of a conflict with Python keywords in Nirum/Targets/Python.hs.

[dahlia]

An array of strings (e.g. ["sample", "example", "nirum"]) would be better than a string for keywords.

I named keywords metadata to packageKeywords because of a conflict with Python keywords in Nirum/Targets/Python.hs.

We can resolve a conflict of names without choose a different name. Qualified import is enough:

import qualified Nirum.Package.Metadata as MD

pKeywords :: Code
pKeywords = fromMaybeToMeta $ MD.keywords metadata'

[earlbread]

@dahlia Thanks for the advice. I'll fix it soon!

[earlbread]

@dahlia I changed keywords metadata to an array. Thank you!

[dahlia]

Thanks!