Add support for user specified context types

[clnhlzmn]

In the generated struct <machine> { ... } and struct <machine>_event { ... } there is a void *ctx. The user should be able to specify the type of ctx. Maybe it could be passed on the command line (java -jar makina-compiler --machine-context struct my_type input.mkna) or the generated files could use a macro for that variable's type which would have to be defined by the user.

[clnhlzmn]

Added in bba922a.