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 *,300
size *,30

Also added, perhaps less useful, range test for width testing, for
example

size <min-width>:<max-width>,<height>
Merge pull request #5 from redskyit/development

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