adds focusgroup attribute basic support

[mezhnin]

Notice: I had to slightly increase size-limit value in package.json to pass the tests.

[ai]

We also need:

1. To add to test/demo
2. Add section to README.md » Focus Group. You can just add an example, I will write more text.

[mezhnin]

We also need:

1. To add to test/demo
2. Add section to README.md » Focus Group. You can just add an example, I will write more text.

There are 3 demo blocks with focusgroup attribute in test/demo (inline, block and no value). I don't know what else to add.
I will update Readme in a next PR.
Thanks for the merge.

[mezhnin]

We also need:

1. To add to test/demo
2. Add section to README.md » Focus Group. You can just add an example, I will write more text.

#47