Ignore SVG shape calls

[wyoung]

The following line causes an error from pyScss:

clip-path: circle(10px at center);

This is legitimate CSS, not a call to a SCSS function. The pyScss compiler should recognize such SVG shape calls.

The current version does correctly appear to pass such lines through untouched, but the resulting error should also be suppressed:

ERROR: Function not found: circle:1