Add `helpers` module for common behaviors

[tadmccorkle]

Problem

Many people are accustomed to certain behaviors when editing markdown files; for example, adding a list item on <CR> when the cursor is at the end of an existing list item (see #12). Some of these are not well supported by the existing plugin commands or API.

Solution

Add a helpers module that can be used for common behaviors. This module will be intended for use in the on_attach callback of the plugin configuration when setting up mappings.

The initial module implementation can include a function that handles a <CR> mapping for inserting list items. The behavior of this mapping should be:

• If the cursor is at the end of an existing list item, add a new list item.
• If the cursor is at the end of an empty list item, remove the empty list item and add a new line.

If there are other common behaviors for inserting list items on <CR> that conflict with the above, the helper function can instead return a specialized function based on options provided when setting up the mapping.