Fix appveyor.yml problem

Fixes an issue discovered while debugging why the Windows CI build kept failing; turns out the path to the extracted TeX Live installation ended up being `C:\texlive\texlive`.
:lipstick:
Use `exec` callback instead of event
:white_check_mark: Improve specs

* Extract common spec setup into utilities module.
* Improve `LogParser` spec reliability by parsing a log file generated
  on the system executing the spec, which means the parser output path
  should always return as expected regardless of platform.
:memo: Update README.md

[ci skip]
:lipstick:
Refactor specs

* Introduce new `Builder` specs.
* Extract utility functions into `spec-helpers` module.
Implemented open-pdf support.

Presently the spec only tests for correct handling of errors. Do not really have idea what to test in case of success.
Simplify code
:feelsgood: Add coffeelint.json and fix problems
:see_no_evil: Remove default linting rules
:boom: Rename 'pdf-opener' to 'opener'
Make opening the result after build configurable
Update README.md prior to v0.12.0
Simplify spec :sparkles:
Add test cases for missing opener support
:lipstick: