Allow matching any line number of example in filter

[matthewmcgarvey]

Fixes #19

This allows calling crystal spec file_path:line_number with any number that falls within the specific spec's boundaries.

Caveats

• There is currently one failing test where the end line number is wrong if there are comments in the last lines of the "it" block. I can't figure it out since it works on a test file but not with this code. My theory is that there is actually a bug in crystal from the multiple macros that the blocks get passed down through, but I have no idea how to track it down.
• You can only specify line numbers that fall within the specific "it" block. You cannot put a line number of the context block and expect all the specs within it to run. From what I've seen, only the it block sources get passed in so there'd have to be a way to hold a reference to it's parent so that it could check if the number falls within its range (and that parent would also have to hold a reference to its parent, etc)

[icy-arctic-fox]

• You can only specify line numbers that fall within the specific "it" block. You cannot put a line number of the context block and expect all the specs within it to run. From what I've seen, only the it block sources get passed in so there'd have to be a way to hold a reference to it's parent so that it could check if the number falls within its range (and that parent would also have to hold a reference to its parent, etc)

The example's group can be retrieved by calling example.group. The example group has its own source (example.group.source). That could be checked along with parent groups. That might be outside the scope of this PR/issue. Up to you if you want to implement that.

[matthewmcgarvey]

I made a new issue to encompass the line numbers for groupings of specs https://github.com/icy-arctic-fox/spectator/issues/24.

With the one problem of specs with comments/newlines at the end of the block, I think this is ready.