Add support for durations

[ctron]

It would be great to have support for durations:

use std::time::Duration;

#[derive(Envconfig, Clone, Debug]
struct Config {
   #[envconfig(from="TIMEOUT")]
   pub timeout: Duration;
}

Using as:

TIMEOUT=1ms # one millisecond
TIMEOUT=1s # one second
TIMEOUT=1m # one minute
TIMEOUT=1h # one hour
# TIMEOUT=1 # fail with error: no unit

[greyblake]

@ctron The use case is valid.
However, Envconfig relies on FromStr, which is simply not implemented for std::time::Duration.
What I suggest to workaround this is to use humantime crate.

Please reopen the use if it does not solve your problem.