Invalid heading level generation

[muan]

From #20. Reference: https://spec.commonmark.org/0.29/#example-33

More than six # characters is not a heading

Here's an example test for this:

it('does not insert header for invalid level', function() {
  setVisualValue('|title|')
  clickToolbar('md-header[level="10"]')
  assert.equal('|title|', visualValue())
})

FAILED TESTS:
  markdown-toolbar-element
    after tree insertion
      header
        ✖ does not insert header for invalid level
          HeadlessChrome 76.0.3809 (Mac OS X 10.14.6)
        AssertionError: expected '|title|' to equal '########## |title|'
            at Context.<anonymous> (test.js:509:16)

cc @mintbridge @koddsson