Add custom parser support to Env provider

[felipou]

Add the Env::parser function to setup a custom parser for environment variable values, allowing the usage of other formats like JSON or YAML.

[calavera]

@SergioBenitez I would use this if it was available in a release. I was just wondering how to pass arrays of values through environment variables.

[joaopcm1996]

Any plans to merge this @SergioBenitez ? In dire need of it as well

[lmmx]

The test needed a cfg(gate) but otherwise looks fine

Merged on figment2 fork:

• https://github.com/lmmx/figment2/pull/5