INDENT rule doesn't handle "extend" properly

[pascalmr]

Issue #27 introduced support for the "extend" keyword.
Unfortunately the indent rule doesn't handle it correctly. It seems to expect no indentation for the extension content (https://github.com/googleapis/googleapis/blob/master/google/api/annotations.proto):
[google\api\annotations.proto:30:3] Found an incorrect indentation style " ". "" is correct.
[google\api\annotations.proto:29:3] Found an incorrect indentation style " ". "" is correct.

[yoheimuta]

@pascalmr Thank you for your report.
It appears to be sure in need of the fix.

[yoheimuta]

I fixed it by #68.

[yoheimuta]

@pascalmr Please let me know if you can still experience the problem.