Fix 'haxelib path lib' for lix-pm installs

[dpomier]

This PR makes possible to run 'lix run lime', which internally seems to rely on the command 'haxelib path lime'.

The changes should not affect existing scripts:

• hxp.System.runProcess has an additional argument allowNonExecutables. If true, the latter changes the way sys.io.Process is instantiated by using null instead of args. This allows running shell commands that are not executables, as well as executables.

An alternative to the above approach would be to do as hxp.System.runCommand does, that would be something like:

// automatically allow non-executables commands when `args` is an empty array
args.length != 0 ? new Process(command, args) : new Process(command);

This would not require a new argument, but some users might experience different behaviors; For instance on Windows System.runProcess("dir", []) will behave differently.

[dpomier]

Closed in favor of #18