Bump version to 0.3.6

* Fix the bugs that were introduced in 0.3.5 that caused the file to be renamed
  incorrectly.
* Add `--downcase` option to make lower case each word in the filename
* Add `--capitalize` option to capitalize each word in the filename
* Make `_` underscore the default value for `--sep-string`
* Remove less used optons
  - Remove `--inc-words` option!
  - Remove `--exc-words` option!
  - Remove `--ignore-case` option!
* Consistently use double quote for string
* Update Coverall configuration
  - add `.coveralls.yml`
  - adjust the settings in `test/test_helper.rb`