Updated extraKeys (bold and italic) and added new shortcut (for span tags)

[RKuerten]

So, this pull request is to address #1148. I also added a shortcut to wrap a selection in span tags (I chose Ctrl+F1 because it don't think it messes with any browser/system shortcuts afaik, but I'm open to suggestions). With the new renderer, it may be interesting to add a shortcut for the new divs ({{ }}) as well.

I can't test if the Cmd+B/+I/+F1 shortcuts work (I'm on Windows), but there's no reason for them not to work.

I left strikethrough key maps out, but I can add them too.

[ericscheid]

I would caution away from mapping Cmd-[x], as those are the keys the browser uses for commands. Right now, Chrome doesn't have any key bindings for cmd-B or cmd-I, but it does have a mapping for cmd-S.

So, while we could be cautious and judicious with selecting specific keys for homebrewery (to avoid clashes), we will end up having to choose some odd-looking alternatives (e.g. we can't use cmd-S for strikethru so we to choose something else which is less obvious). [Actually, we've already hijacked cmd-S from the browser, invoking the "save edits of brew" function.]

Although there are no characters for ctrl-{ (like there are for ctrl-B), JS doesn't detect characters, it detects keys and modifiers. So we could go ahead with ctrl-{ for span (or for {{..}} blocks/spans).

[calculuschild]

This seems fine to me. Other websites (like Github) also have keybindings for Ctrl-I, Ctrl-B for bold, italicize. It's not unusual. CodeMirror even explicitly gives examples of doing this to override existing browser key-combinations. And we do already use CTRL-S for saving; it's a hotkey for most other text editors, so it makes sense to change the default key-combo behavior in that case. The Homebrewery is a web-based App, not a static page in the browser, and many other "text-editor" webapps do the same thing. This is very common practice.

I do like the idea of changing the "span" key combo to use the { button though. @RKuerten Would you make that change and then I can merge this?