[LINT] Exclude directories from CT linting

[Zethson]

ATM some functions check every file for something e.g. leftover cookiecutter statements.

It would be useful to state regexes/lists in the .cookietemple.cfg file, which exclude directories from being picked up by our custom linting functions.

[Zethson]

Test Warnings: 
 https://cookietemple/linting/errors#3 : TODO string found in .travis.yml: uncomment if deploying with travis instead of Github CI
  https://cookietemple/linting/errors#3 : TODO string found in .travis.yml: uncomment if deploying with travis instead of Github CI
  https://cookietemple/linting/errors#3 : TODO string found in publish_package.yml: Configure your secrets to enable automatic deployme..
  https://cookietemple/linting/errors#3 : TODO string found in TemplateLinter.py: Go through all template files looking for the string ..
  https://cookietemple/linting/errors#3 : TODO string found in TemplateLinter.py: if 'TODO COOKIETEMPLE:' in line:
  https://cookietemple/linting/errors#3 : TODO string found in TemplateLinter.py: .replace('', '') \
  https://cookietemple/linting/errors#3 : TODO string found in TemplateLinter.py: .replace('', '') \
  https://cookietemple/linting/errors#3 : TODO string found in TemplateLinter.py: .replace('', '').strip()
  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_name }}

  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_short_descriptio..
  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_name }}

  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_short_descriptio..
  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_slug }}

  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_name }}

  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_short_descriptio..
  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_name }}

  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_short_descriptio..
  https://cookietemple/linting/errors#4 : Cookiecutter string found in 'README.rst': {{ cookiecutter.project_slug }}

[Imipenem]

Closing this in favor of #535