Run code prior to each exercise run

[rly]

Allow pre-defined code to run prior to each exercise run. This would simplify code exercises by hiding things like import statements and other set up steps.

Something like adding precodeblock below:

<precodeblock id="01_02_01" />

<codeblock id="01_02_01">

Review the code in Section 1 if you are stuck!

</codeblock>

with a script like exercises/pre-exc_01_02_01.py