Allow keyboard navigation when capturing tab

[buckett]

When capturing the tab key (enabled by default) this can block user who use keyboard navigation from escaping the editor and is an accessibility issue.

There is some discussion about how this can be handled in:

• https://codemirror.net/examples/tab/
• Keyboard only: how to escape textarea effectively and move to next focusable? PrismJS/live#8
• https://ux.stackexchange.com/questions/39868/accessible-way-to-override-tab-key-in-textarea

The codemirror option of supporting escape, tab seems like a reasonable one (but maybe that's me showing my vi history).

[antonmedv]

What about only capturing tab if something was modified?