Command line flags for extra features

[JustusAdam]

We need command line flags to enable experimental/additional features.
Currently this only concerns the tail recursion feature.

However I would suggest we add a general mechanism to enable features.
I was thinking a -f FEATURE flag. With the command line parser we use this can easily be done and the -f flag can the be supplied multiple times to enable different features.

Example:

ohuac build file.ohuac -f TailRec -f SomeOtherFeature

[JustusAdam]

Actually, I just realized that I used -f for the format for code gen in the build command.
I think we should rename the current -f command to -c (for code-gen) and use -f for feature instead