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 the 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.

**Modified Behavior**
The buffers after `yw` would be `$varname` and `long-desc-class-name` respectively.