A way to define variables on command line

Hello,

It would be nice that ppx-optcomp accept some `-D var` option to define a variable on command line, allowing, for example, to use such a flag to compile in {test|debug|...} mode or not. This would allow to use
````ocaml
#ifdef testmode
...
#endif
````
without having to `#define testmode` or not in each file.

The issue names no files or tests. Start at the ppx-optcomp command-line entry point and inspect how #define and #ifdef variables are currently handled. Done means a -D var option defines a variable for conditional compilation, allowing the shown testmode example without editing each source file.

Written by the indexing model from the issue text.