Fix pluralization for the word 'lens'

[fywk]

What is being done in this PR?

Fixes #78

Add pluralization rule for the word 'lens'.

What are the main choices made to get to this solution?

Without this rule, both the Singularize and Pluralize functions would return incorrect results as described in #78.

List the manual test cases you've covered before sending this PR:

Ensured the fix is tested via singlePluralAssertions in flect_test.go.