Add regex support for optional dependencies

[NinFreakinJa]

Additional regex to support the specification of optional dependencies/extras for a package. Current behavior will match the extra name within the '[]' rather than the package + specified extra.

[billallen256]

Note that this still allows mistakes like abc[DEF HIJ, which will return three package names. However, short of implementing the full grammar, I'm not sure of a way around this. Also, the intent of the regex is to prevent malicious strings, not mistakes. Curious if there are other thoughts before merging this though.

[NinFreakinJa]

Note that this still allows mistakes like abc[DEF HIJ, which will return three package names. However, short of implementing the full grammar, I'm not sure of a way around this. Also, the intent of the regex is to prevent malicious strings, not mistakes. Curious if there are other thoughts before merging this though.

That is a good point. I also don't immediately see a way around this without even further extending the regex to check for when whitespace is and isn't allowed in the grammar.