Lesson 7: markdown tasks have problematic tests

[mglukhikh]

One of known problems is overlapping markdown tags producing non-valid HTML <i><b></i></b>. May be something others. Anyway, we should either fix these problems or just delete these tasks and replace them with something else.

[mglukhikh]

Related issue: #72

[mglukhikh]

Other problem here: sometimes markdown "brackets" have no pair.

[mglukhikh]

I suggest here to generate some random element tree, then convert it to both HTML and markdown. Consistent basic tree should produce consistent HTML, so we'll have no problems from the issue.

[mglukhikh]

Overlapping tags problem should be fixed.