Add wildcard support for at *,y and size *,h tests

Due to changes in browser font rendering, sizes and position of some
elements (e.g. a span) can vary, so add support for a wildcard x
position for an “at” test, and a wildcard width for a “size” test.
Examples:

at *,<y>
size *,<height>

Also added, perhaps less useful, range test for width testing, for
example

size <min-width>:<max-width>,<height>