Configurable time-out for network / IO operations in get-resource

[mjul]

In order to create robust code it would be great to have a configurable timeout for network operations (and possibly also other IO operations that might hit a network barrier or be unavailable), ie. in the get-resource multi-method, especially when hitting a URL or URI.

It also (indirectly) applies to html-resource and xml-resource.

[cgrand]

I don't want to grow get-resource in a full-fledged http-agent. I should make this pluggable though.

[cgrand]

@JustinIAC evaluates whether we could use clj-http (or other) as a dep for the http-client (and make the client an option as the parser is).

[cgrand]

@JustinIAC is looking into it