env-prefix-map

Function that:
* gets all the keys with given prefix
* removes that prefix
* (by default) lowercases the keys

Usage

The function is provided to create map out of env variables. Eg, for env variables:
``
PLAYS_CHELLO='Mark'
PLAYS_VIOLIN='Mary'
PLAYS_GUITAR='Jimmy'
SOME_VARIABLE='x'
`

the call

`
require('env-prefix-map')(process.env, 'PLAYS_');
`

would produce a map of following form:
`
{
chello: 'Mark',
violin: 'Mary',
guitar: 'Jimmy'
}
`

$3

#### Change keys

One can differentiate the way key is being tranformed (toLowerCase by default):

`
require('env-prefix-map')(process.env, 'PLAYS_', {keyFn: k => 'plays ' + k});
`

will produce:
`
{
'plays chello': 'Mark',
'plays violin': 'Mary',
'plays guitar': 'Jimmy'

}
`

#### Change values

You can also transform values:

`
require('env-prefix-map')(process.env, 'PLAYS_', {valueFn: v => v.toUpperCase()});
`

will produce:

`
{
chello: 'MARK',
violin: 'MARY',
guitar: 'JIMMY'
}
``