Conversion to Swift 4.1

Updates to support new syntax for String in Swift 4 and using compactMap(_:) in Swift 4.1

Get ready for rush hour.

Some optimizations made when making tokens from a single tokenizer.

Now with more expressive tokens

Switch from token tuple to token type protocol (#5)

Switch from tuples to TokenType's

Safer tokens with single tokenizers 🤕

Add default tokenizer type protocol (#3)

* Add DefaultTokenizerType.

* Update tests.

* Update documentation.

Token -> Tokenizer

Change from token type to tokenizer type (#2)

* Update API

Switch internal terminology from use of `Token` to `Tokenizer`.
Change external API from `matches(:)` back to the original `tokens(:)`.
Add a convenience method `components(:)` that returns just substrings
rather than array of `Token`.

* Update file name.

* Update README to show `Token` type.

* Include `import Mustard` in code snippet.

Initial feedback

0.1.0

Fix typos and readability in examples.