Add istanbul coverage report generation

**WIP! DO NOT MERGE!**

Usage:

```
# Run specs and produce coverage JSON objects for future report generation
# In case of "desktop.specs" you could use any build target, 
# like "desktop.specs/button" or "desktop.specs/i-bem__dom"
› ISTANBUL_COVERAGE=yes bem make desktop.specs
....
# Generate HTML-report using coverage data produced with `bem-make`
# from previous step
› istanbul report html
› open coverage/index.html
```