filter process.env by regexp

adds match option for pattern matching when filtering process.env variables... for example

``` js
config = nconf.env({
    separator: '_',
    match: /^prefix/,
    whitelist: ["cumbaya"]
})
```

in this case all the variables that match the regexp are whitelisted... whatever is in whitelist still gets loaded

this is some handy dandy project.. awesome! thanks!