Change Lookup to take an argument

Also Rename OnlineThesaurusLookup to OnlineThesaurusCurrentWord,
and allow the user to search for arbitrary words using
OnlineThesaurusLookup
Update documentation for functions
Rename OnlineThesaurusLookup to Thesaurus
Add back OnlineThesaurusLookup
Merge pull request #4 from justincampbell/functional-lookup

Change Lookup to take an argument; added OnlineThesaurusCurrentWord and a short command Thesaurus