Add functions for prefixed environment variables

[dkaslovsky]

Thanks so much for this repo, I've found it extremely useful!

This PR adds functionality for easily working with environment variables that share a common prefix, as is often the case for the configuration of an application. For example, consider (modified from the README in the current master branch):

type ConfigDatabase struct {
    Port     string `env:"PORT" env-default:"5432"`
    Host     string `env:"HOST" env-default:"localhost"`
    Name     string `env:"NAME" env-default:"postgres"`
    User     string `env:"USER" env-default:"user"`
    Password string `env:"PASSWORD"`
}

The environment variables serving as the configuration for the database application might share a common prefix such as DB or APPNAME (e.g., DB_PORT, DB_HOST, etc). To cleanly and easily handle this case, rather than repeating the prefix in the struct tags (hindering readability), we add the functions:

• func ReadConfigWithPrefix(path string, prefix string, cfg interface{}) error
• func ReadEnvWithPrefix(prefix string, cfg interface{}) error
• func UpdateEnvWithPrefix(prefix string, cfg interface{}) error

so that the prefix is specified once on read. Following the example above, we read the prefixed environment variables as:

var cfg ConfigDatabase

err := cleanenv.ReadEnvWithPrefix("appname", &cfg)
if err != nil {
    ...
}

For completeness, this PR also adds the following description function:

• func GetDescriptionWithPrefix(cfg interface{}, prefix string, headerText *string) (string, error)

This functionality is intended to work similar to the prefix functionality of kelseyhightower/envconfig

Thanks for considering this PR, and I'm more than happy to iterate on any feedback you might have.

[codecov]

Codecov Report

Merging #65 (ee445ab) into master (ce3ee7f) will decrease coverage by 0.14%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   89.79%   89.65%   -0.15%     
==========================================
  Files           1        1              
  Lines         245      261      +16     
==========================================
+ Hits          220      234      +14     
- Misses         17       19       +2     
  Partials        8        8              

Impacted Files	Coverage Δ
cleanenv.go	89.65% <91.30%> (-0.15%)	⬇️

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[batazor]

@ilyakaznacheev Can we add this feature?