Implement --define-mode and -S options

[aradi]

The --define-mode option controls, whether values in -D options are evaluated or treated as string literals. The -S (set) option initializes variables with expression evaluation (independent of the --define-mode settings).

Closes #32

[zerothi]

Thanks for this!

Some comments:

• I still read -S as --string, but I might be biased from the prior discussion.
• With this, I would argue one should have both the string-literal option, and the expression-literal option.

Effectively I would go for:

• -E/--define-eval VAR=value
• -S/--define-string VAR=value
• -D/--define VAR=value (follows --define-mode)

In this way, you will always have the full control, while one could do with 1 less option. But I think -S for --set is a bit ambiguous in terms of what set means. Eval is much clearer IMHO. So if you don't want 3 options, I would go for -D and -E (--define-eval).

[aradi]

@zerothi Thanks for your insights. I see the point. I have updated the PR accordingly. I'll still digest it a bit before merging, but I think, that's the most consistent solution, indeed.