Add the possibility to get a single value

[vncntd]

Hey !

Using the library for a project I am looking for a way to be able to get a single value from the config and potentially load a different file following the returned value.

I think it should be really interesting to have a way to get this value without loading the whole config on a struct or having to create a struct with just one field to get it.

After taking a quick look at how you implemented the backends I think it should not be that hard to add a method like :

func (l *Loader) Get(name string) interface{}

or maybe something typed with a method for each ? (String, int ...)

WDYT ? I should take a look and try to work on it if this is something you are interested by.