Add support for dotenv (Shell)

[smola]

Fixes #4873

• Associate *.env, .env, .env.example and .flaskenv to Shell.

Add samples:

• .flaskenv from https://github.com/stripe/stripe-payments-demo/blob/70ba859bb68a650b35b6e85ddc889ad143055170/server/python/.flaskenv (MIT License)
• .env.example from https://github.com/proasis/api-rest/blob/670123448b19229677f0710751b2ef4e808f9bd3/.env.example (MIT License)
• .env from
  https://github.com/rabbitmq/tgir/blob/4299d2513dad4bbdae87b107c1f1ec0b14fc04a1/.env
  (Apache 2 License)
• default.env from
  https://github.com/diegoalmesp/trello-eureka/blob/43a4488f6df50be2a17cd9495ce29de6e4192b14/client/default.env
  (MIT License)

Description

dotenv files is a convention to use a subset of shell (commonly known as dotenv) to setup an environment.

The modern convention probably originates from the Ruby world in 2012 (see bkeepers/dotenv) and the supported syntax is roughly the following (although packages for different languages may support a more limited subset):

# comments
KEY=value
KEY="value"
export KEY=value
KEY=${variable}
KEY=$(command)

Anyway, you may find lots of samples in the wild using full Bash syntax (see the links to GitHub Search below).

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • .env: https://github.com/search?q=filename%3A.env
      • .env.example: https://github.com/search?q=filename%3A.env.example
      • *.env: https://github.com/search?q=extension%3A.env+%28port+OR+host+OR+path+OR+user+OR+pass+OR+export%29
      • .flaskenv: https://github.com/search?q=filename%3A.flaskenv
  • I have included a real-world usage sample for all extensions added in this PR: (added 3 fixtures)
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

You may find some counter-examples, but not with wide usage as far as I see:

• Dockerfile.env (Dockerfile): https://github.com/search?q=filename%3Adockerfile.env
• Make.env (Makefile): https://github.com/search?q=filename%3Amake.env&type=Code

[pchaigno]

One nit below. Usage looks good, so LGTM otherwise!

[pchaigno]

Would it be possible to find sample files from "real" applications here instead of using fixture files? Fixture files are not used to train the Bayesian classifier, so I'd really prefer to avoid them as much as possible.

[smola]

@pchaigno Ok, I'll add some!

[smola]

Please, see the discussion the issue: #4873
Some languages may have syntax rules are not strictly a subset of Bash. At least for Node.js, where VAR={"some": "json"} is valid, but not in Bash (or any other mainstream shell). So we may still have some .env files with some highlighting errors after this change. Unfortunately, I don't see a better alternative, since using a dotenv-specific grammar may incorrectly highlight the files for many other languages that allow different extensions, or for bull Bash itself.

[smola]

To add some more fun, the Ruby library supports YAML-like variables: https://github.com/bkeepers/dotenv/blob/master/spec/fixtures/yaml.env