Add more NET standard support

• add 2.1 and 1.6 support

v2.1.1

• Split the dotenv package into component libs

More descriptive exceptions

Add the errant path to the exception message

Performance improvements

• Switch to using a singleton instead of a static class
• Fix issue with env vars that have an equals ("=") sign

add injection support

Merge pull request #3 from bolorundurowb/develop

release: add support for injection