allow modifying cookie settings

[ovalek]

Allow modifying cookie settings by passing closure with CookieBuilder to cookie_adjust in TokenSigner builder.

As mentioned in #24, we could change cookie settings when creating the initial access_token or refresh_token, but we couldn't easily change the access_token when it is refreshed. This PR allows to adjust cookie settings (e.g. Path) as follows using cookie_adjust():

TokenSigner::new()
    .signing_key(key.clone())
    .algorithm(Hs256)
    .cookie_adjust(|builder: CookieBuilder| builder.path("/"))
    .build()
    .expect("Error: Could not create a TokenSigner")

[michaelvanstraten]

Would passing an initial CookieBuilder also be fine with you? I think it should offer the same level of flexibility with a bit less exposure in the interface. Please speak out if you have a use case where a function is required; it could be that I don't see the full picture here.

[ovalek]

As long as I can set path, http_only and same_site parameters for the refreshed access_token I'm fine with it. I added new commit that adds bare CookieBuilder as a parameter. The Cookie::build method requires name and value, that have to be overriden. The builder doesn't have methods to set name/value, so I set these on the final Cookie.