Make everything configurable with env variables

[php-coder]

To comply with 12 factors (see https://www.12factor.net/config), let's make everything configurable with env variables.

The following line should be modified to not hard-code a host and port:

mystamps-country/main.go

Line 33 in 85bf3df

dsn := fmt.Sprintf("%s:%s@tcp(localhost:3306)/%s", user, pass, dbName)

[php-coder]

I looked at https://github.com/kelseyhightower/envconfig and I've decided to not use it for the following reasons:

• there should be a good reason for introducing a new dependency
• our requirements are small at this moment and they can be solved with a little custom code (+ https://golang.org/pkg/os/#Expand)
• while this library provides some benefits (variables can have a static prefix, auto transformation from string to a particular type, usage helper) we won't benefits much from them as they aren't required in our case
• the library has open issues and pull requests with no response for some time and it seems non-maintained

[php-coder]

One more project to consider: https://github.com/ardanlabs/conf