Add support for configuring a timeout

[banool]

I'm currently trying all kinds of hacks to make it possible to call self_update with a timeout, but it feels like it would be easiest if the library allowed either / both of these:

1. Allow configuring the timeout when building the Update object.
2. Allow passing in your own reqwest Client.

Of course this might be the better long term fix: #44. But the above two things are easier for now.