Isolate Input Traversal

[profbrandon]

As suggested in #10, the current status of the rich text editor is that it primarily deals with tree traversal, with different markup nodes labeled as positions in the tree. This should be formalized. The rich text editor should be refactored to only account for this functionality, and functionality related to input and rendering should be dealt with elsewhere.

As a result, it should be renamed to something like "traversal-input" or "tree-editor" or "tree-traverser". Additionally, the interface that it exposes needs to be pinned down.

[profbrandon]

Things the user should provide:

• a keystroke processing callback
• a rendering callback

Possibly like this:

const processKeystroke = (key, state) => {
    ...
    // return action
}

const renderTrees = (state) => {
    ...
    // return parent element
}

The tree traversal input should probably offer several kinds of actions specific to itself.