implement http authorization

[Kanezoh]

want to cover http authorizations, at least basic auth first.

The original mechanize uses #add_auth to add authorization info to agent and saves it in AuthStore.

https://github.com/sparklemotion/mechanize/blob/4a0dfe57123ff78066d72fe1106db3abc547cb65/lib/mechanize/http/agent.rb#L208

In Crystal, there is #basic_auth method to add authorization info to http client.
https://crystal-lang.org/api/1.2.2/HTTP/Client.html#basic_auth(username,password):Nil-instance-method

It is an instance method, but now mechanize.cr uses class methods to send http request like HTTP::Client.get~, so we may need to rewrite it to use instance method like client.get~

[Kanezoh]

#22