Parse function names with nested vars

For instance, pkg_postinst_${PN}

Fixes #3
Remove "keepend" and "excludenl" directives

It looks like these were never actually used correctly, and were doing
more harm than good. "keepend" on bbString, for instance, prevented
proper nesting of ${@python} in strings. Similarly, a balanced pair of
{ } braces inside a shell function would force the function to terminate
early if the closing brace was on its own line.

So far I've seen absolutely no negative consequences from removing
these, but a bunch of positive consequences.

Fixes #1