Kompari xtask (html report, interactive test blessing, dead snapshots detection)

[spirali]

Adds command cargo xtask that allows to:

Creating report

cargo xtask report

creates HTML file with report containing snapshots that does not match

Interactive test blessing:

cargo xtask review

stars a local web server with the following page:

(screenshot obtained by putting an artificial error into each snapshot test)

Detecting dead snapshots

cargo xtask dead-snapshots

This PR fully replaces #185