Support Additional `Selector`s with Key Value Pairs

[remkade]

Right now the parser understands key value pairs that are not simple equality. For example: [subdir != "win-64] is valid and understood. However, you can't yet do anything with it yet. Lets support this properly and make the matching logic understand it.

Examples

Here's some possible sensible use cases:

[license != "GPL-3"]
[build_number > 2]
[subdir != "win*"]

Here's some that shouldn't work:

[license >= "GPL-3"] # doesn't make sense
[build >= "hdfdf_py39" ] # No way to do greater than on a string like this with out causing surprises
[sha256< "abcdef12345"]

[razzlestorm]

If we are adding equality checking for things like license and sha256 and the like, does this mean we're going to have to add those attributes to the MatchSpec struct? I've gotten rusty in my Rust, so I apologize if this is an obvious question.