Normalize export style in problem.md

[Sequoia]

also problem.md markdown cleanup
closes #45

What I did

• Fixed up export style as discussed
• Made h2 style consistent (took out superfluous wrapper hash marks: markdown parser sets these apart in blue already)
• Removed superfluous newlines at start/end of code blocks
• I moved "boilerplate" section to the end of each problem.md for consistency

Ongoing issues

• Some of these are way too long (part of reason I shortened h2 syntax)
• Style is inconsistent: some start with # Task, some have lots of intro etc.. this should be standard (is there a styleguide for this?)
• order of sections should be consistent

[Sequoia]

Made # Task h1 consistently, every other heading h2. It doesn't show up differently in terminal right now but "more semantic" 🐩

[timoxley]

Very good.

[timoxley]

this has been on my todo for a long time.