Partial fixes for IE11 compatibility

Adds the parameters required in IE to ```document.createTreeWalker``` for the following files: 
```
src/plugins/core/events.js
src/plugins/core/formatters/html/enforce-p-elements.js
src/plugins/core/inline-elements-mode.js
src/plugins/core/patches/commands/insert-html.js
```
Also, in ```src/api/command.js``` the ```document.queryCommandState```/```document.queryCommandEnabled``` calls can cause 'unsupported command' exceptions in IE, so I've wrapped them with try/catches.

Some (or maybe all) of these changes are covered by #218 and/or other of @daniel-nelson's pull requests, but since they seem to be mixed up with unrelated changes, I've decided to prepare this pull request for the some of the more straightforward issues so that they can be easily merged, and so we may be able to / can begin fixing each of IE's problems one by one.
What does IE do if this command is not available. Not really a fan of just catching errors and throwing them away.