Add -package_format to customize the generated package name

[dr-dime]

Previously -package must be explicitly defined or it will default to mock_${name}. To generate a mock file that sits next to the package files, eg. as part of the test, one must extract the package name first. Which is not scripting-friendly.

After this change, we can use -package_format to customize the generated package name. So that a script can run mockgen without knowing the package name first.

Fixes #631

[dr-dime]

@codyoss friendly ping.

[dr-dime]

@codyoss friendly ping.

[yuamera]

I also have the same needs, I hope to use it as soon as possible, thank you!