Allow plugins to change the default pause

[twitwi]

Some frameworks don't need any pause between slide export (i.e. running with -p 0 works fine) and the default value of 1000 ms pause is slowing down the export for users that are unaware of the -p option.

It would be useful if an API could be used by plugins to change the default pause (still making it overridable via the command line for special cases).

The same might be for the --load-pause.