Add static interface implementation check

[breml]

With the flag staticcheck set, an additional line of code per interface is
added to the generated mock file, which allows the go compiler to statically
check if the mock implements the mocked interface.

[breml]

@matryer rebased this PR, so it should be ready to be merged.

[matryer]

@breml I'm not sure we need this, wouldn't that basically happen when you come to use the mock anyway? I.e. if you're passing it into something that accepts the original interface, it'll be checked then?

[breml]

@matryer In principle I agree. I see two advantages of this staticcheck:

1. The compile error points you the the file where the mock is defined, instead of the file where the mock is used.
2. If the mock is passed to a function, which accepts a narrower interface (or even the empty interface) and then an assertion is used, with the staticcheck, the compiler will still complain, whereas without the staticcheck the runtime will panic or (hopefully) a test case will fail, because a different code branch is taken after a type switch or a type assertion. In either case the problem is only visible when executing the code, whereas with the staticcheck already the compiler (and with this my editor on save) is able to detect the problem.

For me, the staticcheck adds a second line of defense, which can be achieved with a single line in a generated file. This looks for me as well worth.

[breml]

@matryer have you considered my comment above?

[matryer]

@breml If this is worth having, then we could always include it rather than it being configuration?

[breml]

@matryer I updated the PR. Now there is no longer a flag, the additional static check is added to every generated mock file.

[breml]

Rebased, now TravisCI is happy.