Add an --unstable flag for installing unstable releases

[tobil4sk]

Right now, haxelib install ignores unstable releases (alpha, beta, rc) when running:

haxelib install library

The only way to install a pre-release version is to manually check for it and install it specifically:

haxelib install library 5.0.0-alpha.2

We could add an --unstable flag to allow installation of the latest unstable release via

haxelib install library --unstable

We could add it to haxelib update as well, but maybe only when updating a specific library, rather than when doing a full update of all libraries?