Allow `regenerateDelay` to be user configurable

A new configuration option `regenerateDelay` has been add. Defaulting
to 100, this option specifies how long docpad waits after a source
file has been changed before using it to regenerate a page.

In situations where source files (especially large files) are being
updated over the network (e.g. via FTP) it can be useful to increase
this option: not doing so can cause files to be partially rendered due
to the fact that a watch and subsequent render are triggered *before*
the file has finished being written to.