Add equal-sign based parsing for flag based args

[cameronmore]

Functional Requirement:

• Parse flag-based arguments that use the equal sign syntax such as: —style=MLA
• Allow users to define a set of possible values such as: [MLA, Chicago, APA]

Implementation:

• Add the necessary code to the .process() method called by the ArgManager struct, alongside the other code that parses the arguments
• Do not change the existing code, this should ‘in theory’ be an additive change
• Allow users to mix equal-sign based args and non-equal-sign based args at the same time (like —style=MLA —output OUTPUT_LOCATION.txt)

[cameronmore]

Looking into this more, to check whether a flag is being used with a “=“, it may require a runtime concatenation of a string, which (as of 0.14.0) requires memory allocation (std.fmt.allocPrint). To use this would start bringing in hidden allocation, which is not very desirable.

Maybe looking into a rewrite from a comptime perspective might solve this problem, but this is a consideration for a current implementation. Would need user feedback and user preference, thought I do like to have no hidden allocation.