Fix issue #240: Multiple long option names / aliases (#349)

* Fixes #240: Multiple long option names / aliases

* We now use a vector of long option names instead of a single name
* When specifying an option, you can provide multiple names separated by commas, at most one of which may have a length of 1 (not necessarily the first specified name). The length-1 name is the single-hyphen switch (the "short name").
* Hashing uses the first long name
* Option help currently only uses the first long name.

Author: eyalroz

CHANGELOG.md

@@ -7,6 +7,7 @@ options. The project adheres to semantic versioning.
 
 ### Added
 
+* Support for multiple long names for the same option (= multiple long aliases)
 * Add a `program()` function to retrieve the program name.
 * Added a .clang-format file.