Validate non empty string segment

[crypticmind]

Changes

• Upgraded elm-test since I was getting the error I cannot find module 'Test.Runner.Failure'. I followed the comments from here.
• Changed the string parser to validate the string isn't empty.
• Added test case.
• Updated the doc for custom to remove the string example since it was no longer accurate.

[crypticmind]

Hi @evancz
Could you please chime in here?
I'd like to know if this PR is still relevant or I should forget about it and delete my fork.
Thank you!