Add data to singular words with no plurals

[Commandtechno]

pluralize.singular('data') returns datum because of the rule here: https://github.com/plurals/pluralize/blob/master/pluralize.js#L385

This PR just adds data to the list of singular words with no plurals because I don't believe data has a plural form

Closes #145

[Commandtechno]

That was an accident

Anyways, I removed agenda from the list as well as mentioned in the original issue because agendas is its plural form