Support escaped backticks in template literals

This is a totally valid use but since the lexar just looks for the next
backtick as a marker for the end of the literal, things fall apart
quickly.
Process all escape sequences in template literals

This repurposes the code the that was previously only for StringLiterals.

Closes #2001
Stop sending W044 ("Unnecessary escape sequences")

It's not a very useful warning
Implement automated code coverage reporting

- Collect unit test coverage using the Istanbul module.
- Update the CI scripts to report this data to the Coveralls service so
  it can be automatically incorporated into pull requests.
- Remove legacy code coverage file and dependency.
Use Shields.io SVG badge for coverage report

Closes #2041
Merge pull request #2039 from caitp/nope

Stop sending W044 ("Unnecessary escape sequences")
v2.5.11
Fix typo in options documentation
Use `path.join` properly and in more places.