Add shrink and optimize commands

[NoSmint]

Add shrink and optimize commands

I felt the available cleaning functions could use some minor improvements
so I added two new commands.

• hist shrink <length> <delta>: Recursively consolidates entries that exceed
  a specified length, matching up to a given delta. Compress works quite
  similar in this regard. The key difference is that compress works only
  on adjacent entries while shrink scans the whole history recursively.
  For this reason the values of length and delta are evaluated before
  the process begins. A warning message will be presented if the process
  might take a significant amount of time due to extensive pattern matching.

• hist optimize <maxlength>: Discards multiline entries and any entries
  exceeding a given maxlength.

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