Add func "LoadString"

[novemberde]

Hi @joho ,
I've added LoadString function to load string when I want to set environment variables from string.

In the case that I'm deploying my service, I don't want to set .env file on Dockerfile.
I'm currently using AWS parameter store. It returns a string value like below

FIRST_ENV="this is first value"
SECRET_VALUE="this is secret value"

So, I need to load from string and add a function to support that.

Please check this PR soon.

Thank you.