Add function and method definition syntax highlighting

This adds `coffeeFunction` and `coffeeMethod` syntax elements which are linked to
`coffeeStatement` and `coffeeObjAssign`, respectively. This ensures that existing
themes will continue to behave as they have, but make it possible to highlight
functions and methods separately ala Sublime Text. For instance one might want to use their theme's function and method highlight colors:

```viml
hi link coffeeFunction Function
hi link coffeeMethod Method
```

Syntax highlighting similar to Sublime Text is achievable with the following:

```viml
hi link coffeeFunction Function
hi link coffeeMethod Function
hi link coffeeObjAssign Statement
```

Based on the efforts of @seanhess, and addressing the issue raised by @bergman with #102 (function calls incorrectly being captured).