Do not automatically add a dot with generator -e option.

Now -e option in generator is more versatile. Especially it avoids
double-dot problem with some build systems.

Given foobar.proto, we now get:
-e .pb  =>  foobar.pb.c (default)
-e _pb  =>  foobar_pb.c
-e ''   =>  foobar.c

Note that if you have used -e option previously, you will have to prepend
. to the argument to get the same filenames as before.

Update issue 122
Status: FixedInGit