Add - and $ as keyword characters

Add - and $ as keyword characters (i.e. remove them
from the list of keyword boundardies). This makes yw more
powerful when used in context of variables, hook names, and css
class names.

Original behavior
Performing yw from the first character of $varname would
copy $ only to the buffer.

Performing yw from the first character of long-desc-class-name would
copy long  only to the buffer.
Merge pull request #42 from salcode/modify-word-boundary-chars

Add - and $ as keyword characters