Make rake new_cop create parent directories

[highb]

Prior to this commit if you attempted to make a cop in a new category,
the generator would error out because the directory didn't exist yet.

This commit adds a check for the existence of the parent directory
in the write_unless_file_exists method and will create the directory
using FileUtils.mkdir_p if it doesn't exist.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
  • No pre-existing issue that I could find.
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
  • Not sure if the maintainers want to deal with FakeFS tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[highb]

It looks like the CHANGELOG is failing specs because it doesn't contain a link. Should I open an issue for what I'm seeing and link it?

[bbatsov]

It looks like the CHANGELOG is failing specs because it doesn't contain a link. Should I open an issue for what I'm seeing and link it?

I guess your handle is missing at the bottom of the changelog file.

[pocke]

👍