Allow nested pause blocks without resuming inner blocks

[geshwho]

Addresses part of #43.

Instead of always setting tc[:prosopite_scan] to true at the end of a pause block, we can remember the previous value from before the pause block and set it to that. This allows nesting of pauses, however deep, without resuming the scan until the outermost pause has reached the end of the block.

[charkost]

Thanks @geshwho !