Add ability to exclude and include methods

[efirs]

It would be useful to allow to exclude some methods of the original struct from generated interface or have only explicitly specified methods be included in the the interface.

[vburenin]

I am not sure you want to include something specifically as at that point generation of interfaces makes little to no sense. However, excluding something maybe reasonable for some cases. Feel free to contribute this change, it should be relatively trivial.

[kazhuravlev]

@efirs check this mr #58

[risicle]

Including methods is still worthwhile especially when the methods have long and verbose call signatures.