Support env variables for config (#111)

[nikonhub]

closes : #111

We could have used os.Getenv() directly as default value for flag.String(). But the are multiple uses of isFlagSet() in the code, which would return false even if the variable is set through the env.

[kehoecj]

@nikonhub Thanks for the PR! Approach looks solid - please address the golangci-lint findings

[nikonhub]

@kehoecj There is a gocyclo error. But I also see another PR opened with the same error on the same function. How should we handle this ?

[kehoecj]

@kehoecj There is a gocyclo error. But I also see another PR opened with the same error on the same function. How should we handle this ?

Ignore - after hacktoberfest I'll go back and do some refactoring to satisfy gocyclone

[kehoecj]

Please add a section to the README.md describing how users can use environment variables in addition to or instead of the command line arguments.

Make sure it's clear that a command line var will override an env var

Tested functionality - everything looks good on that end!

[kehoecj]

LGTM - thanks for the PR @nikonhub !