Add option to bypass dotenv-expand.

[neverendingqs]

If you had a dotenv file with:

456=HI
TEST_1=123$456!@#%^&*()

the final TEST_1 value will become 123HI!@#%^&*(). This is called "variable expansion", and is not always the desired behaviour.

I think I am comfortable with leaving this dotenv-expand on as default. There is no switching cost to change the default value later on, so opting to not do it at this time.

Old Description

The fact that dotenv-expand is used is not immediately obvious. Having it default to off seems to make more sense, especially with #48. This also allows us to change dotenv-expand to an optional(?) peer dependency to reduce the package size.

• Update README to recommend new safe default
• Update README to warn about deprecation
• Add or update toggle to switch behaviour on major version bump