Make the initial retry delay user-configurable

Adds an `initial_retry_delay` option to the Feed object. This is used to
seed the feed's retry_delay whenever it is reset to its initial value.
The previously hard-coded value of 1000ms is now the default.

It is assumed that callers will set initial_retry_delay before starting
the feed with follow(), either in the `opts` constructor parameter or on
the feed object directly.
Document initial_retry_delay option
Merge branch 'initial_retry_delay'