Add expand option to support variable expansion

[jheth]

We are wanting to reference environment variables from existing variables. I've added this as an expand option but could be part of the default behavior if that's desired. This will look for the ${VAR} pattern and replace with the appropriate variable if found.

[ghost]

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Happy coding!

[jheth]

I was hoping to rely on envDefault for HOST and PORT but the package does not call os.Setenv when parsing the config.

[coveralls]

Coverage increased (+0.02%) to 98.765% when pulling 998eb22 on jheth:add-expand-option into bc122c2 on caarlos0:master.

[medyagh]

@jheth
I wouldn't recommend for Default Behavior for security implication without changing the major version to warn the previous users . but I am okay with having this as an optional feature!

For the sake of readability and usability, I suggest being able to specify the expand option like this,

	type config struct {
		SecretKey   string `env:"SECRET_KEY" ExpandVar:True`
	}

instead of

	type config struct {
		SecretKey   string `env:"SECRET_KEY,expand"`
	}

If you could re implement it that way, I am okay with it be merged.

[caarlos0]

We can probably use https://golang.org/pkg/os/#ExpandEnv instead of writing our own function...

os - The Go Programming Language

[jheth]

Thanks @caarlos0 and @medyagh for the suggestions. The simplified the change quite a bit.

[caarlos0]

I think it should be envExpand to be in the same pattern of the other options.

Adding it to the readme as well would be nice =)

[jheth]

Good idea. I've updated and simplified the overall change once again.

[caarlos0]

thanks!

[medyagh]

Looks good thanks for the contribution