refactor dotenv parser in order to support multi-line variable values declaration

[ndeloof]

This is a clone PR for joho#118

[ndeloof]

@x1unix sorry to bother you with this, we'd like to adopt your PR for compose support of dotEnv file, but our oss contribution policy require all commits to be signed-off (git -s).
Would you mind amending your value-separator branch for this? 🙏

[x1unix]

@ndeloof done

[x1unix]

@ndeloof can I contribute any additional fixes directly to this repo? A few projects on my job also depend on my hotfix and there is no ETA when PR will be merged into upstream.

Also, I've changed import url on master branch of the fork and included multi-line hotfix there, so you can also use my fork (version v1.4.0).

go get -u github.com/x1unix/godotenv@v1.4.0

[ndeloof]

@x1unix sure, you can contribute additional fixes, as long as you feel those are relevant in the context of .env support in docker compose (or related) projects.
Please notice we have only kept the "parser" in this repo, as this is not expected to be used as a command line.