Change problem.txt to problem.md for all problems.

[jordanSu]

With problem.txt, problem description in every problem are shown as grey-scale color without syntax highlighting or emphasis highlighting, which may make it hard to read.

This changes update all problem.txt to problem.md and

• Specify the language used (ex. js, sh) for code syntax highlighting.
• Adjust paragraph format to make it easy to read.

[ccarruitero]

Thanks @jordanSu

Seems good, just the file name changed.

[ccarruitero]

@jordanSu I corrected the file name and rebased your changes into master in https://github.com/workshopper/stream-adventure/commit/6704af081794bcec13651a366a9d38e34e53357f

Thanks again for your contribution

[jordanSu]

@ccarruitero thanks for helping me correct that.