Add Parser::parse_short_equals() setting for disabling -o=value

[blyxxyz]

Since v0.2.0 -o=value has been parsed as option -o with value value. This is nonstandard but compatible with clap.

This PR adds a parse_short_equals(bool) method to configure this. If disabled the equals sign is treated as part of the value, i.e. -o with value =value.

This is for example useful for cut's -d option, which is often invoked as cut -d=. That makes it relevant for https://github.com/uutils/uutils-args.

Resolves #13.

cc @tertsdiepraam

[Mango0x45]

Any chance this PR will get merged eventually? For what it’s worth, I am still using this crate actively on v0.1.0 for my projects, as = is very often a valid value in my software

[blyxxyz]

Thanks for reminding me, I'll merge this soon. I'm only debating whether to call it parse_short_equals or rename it to set_short_equals.

Is = theoretically valid in your programs or commonly used? I'd love to have examples besides cut -d=.