Add option to prepend prefix also to custom name taken from tag

[albenik]

By default the struct like:

type Config struct {
	TestEnvFoo string `envconfig:"TEST_ENV_FOO"`
	TestEnvBar string
}

can be initialised from env: TEST_ENV_FOO, TEST_ENV_BAR if no prefix specified,
but from TEST_ENV_FOO, PFX_TEST_ENV_BAR, if PFX prefix is set.

I added ability to enable consistent prefix addition (see tests), with:

envconfig.InitWithOptions(&conf, envconfig.Options{
	Prefix:    "PFX",
	PrefixTag: true,
})

the struct described above will match PFX_TEST_ENV_FOO, PFX_TEST_ENV_BAR envs.

The default behaviour remain unchanged.