Support custom attributes for parched

[onny]

Hello,
I'm using a custom attribute in my PKGBUILDs and want to parse it with parched. I guess this is not supported yet. I wish to use this feature like this:

package = PKGBUILD(filepath)
package.custom('_watch')

Where '_watch' may look like this in the PKGBUILD:
_watch=http://example.com,option1

Hope you consider implementing this :) Thank you!

[SpotlightKid]

You can access non-standard PKBUILD variables via the PKGBUILD._symbols instance dictionary.