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.