Allow defining a custom cookie vault

[aramperes]

First pass at being able to provide a custom Vault<String> to the Requests singleton. I was able to integrate this in my project to encrypt the vault using AES.

cc @sehnryr

Closes #62

[sehnryr]

I don't think there is the need to send an entire Vault<String> object. Wouldn't be better if the end user could just change the path, name and encryption key of the storage file ?

Also, creating an init method in Requests would be better for the UX.