Add a words terminal to improve performance

[jahav]

PR for #163.

[joerivv]

After performing functional tests I can confirm that the performance has indeed improved massively. Excellent!

Two remarks:

First, not all Unit Tests run successfully:

See error of the first one:

Assert.AreEqual failed. Expected:<Parsed>. Actual:<Error>. 
   at XLParser.Tests.WordsTerminalTests.TestTerminalMatching(String input, Boolean caseSensitive, String[] words, Func`2 checkTerm) in C:\Users\joeri\Documents\GitHub\XLParser\src\XLParser.Tests\WordsTerminalTests.cs:line 49
   at XLParser.Tests.WordsTerminalTests.RecognizesAnyWordFromList() in C:\Users\joeri\Documents\GitHub\XLParser\src\XLParser.Tests\WordsTerminalTests.cs:line 15

Secondly, I would suggest to consistently use curly braces { } for all loops and if-statements, just for readability. ReSharper suggests a few more things but I think they can be ignored :)

[WillemJann]

Thanks for the improvements, the code looks good. Together with PR #162 I get a more than 50% performance improvement of the unit test suite on my machine. Good job!

Sorry for the late response to your pull requests. I will try to release a new version of XLParser today, including your improvements.