feat: adding variableExpansion option to turn off variable expansion.

[neverendingqs]

Description

Resolves #90. This pull request adds an option to turn off variable expansion (but leaves it on by default).

Checklist

• CHANGELOG.md
• README.md
• Unit tests
• Consider performance implications
• Consider security implications

Exploratory Test Notes

Using a .env file with the following contents:

INNER_ENV=innerenv
OUTER_ENV=hi-$INNER_ENV

custom:
  dotenv:
    variableExpansion: false

results in { INNER_ENV: 'innerenv', OUTER_ENV: 'hi-$INNER_ENV' } as expected.

custom:
  dotenv:
    required:
      file: true

results in { INNER_ENV: 'innerenv', OUTER_ENV: 'hi-innerenv' } as expected.

No custom.dotenv property inside serverless.yml results in { INNER_ENV: 'innerenv', OUTER_ENV: 'hi-innerenv' } as expected.