Add .splib as alternate extension.

[cascremers]

When we include input files into other .spthy files, we don't want to give them the same extension, because otherwise Tamarin's gui would try to load them and this yields parsing errors, since they are not meant as standalone files.

I therefore suggest we add .splib as a convention and an extension that is also detected as a Tamarin syntax file.

(Currently this cannot be done manually either due to #9 )