Add option for IMMcustom and if possible SDMcustom

[GidonFrischkorn]

Depending on the complexity of the M3custom, I thought about implementing an IMMcustom that allows users to specify their custom activation formulas for different categories and adjust the formulae for the mixture weights accordingly. I have to think a little about how to deal with identifying different categories of swaps (e.g., swaps from WM as implemented currently, and for example swaps from LTM if there is semantic or pre-learned knowledge).

Depending on the complexity of the SDMfull it would be great if there is a possibility to also have an SDMcustom option. But like the IMMcustom this is definitely something for future releases and not from the initial CRAN release.

[venpopov]

Closing this as we now use the "version" argument #178