Add support for inserting new bullets mid-line

[cdalizadeh]

The previous behaviour is that calls to insert_new_bullet() while the cursor is in the middle of the line (in insert mode) do not insert a bullet in the newly created line. This behaviour differs from most word processors, which insert a new bullet when enter is pressed mid-line. This PR modifies insert_new_bullet() to a insert a bullet in the newly created line when called from the middle of the line in insert mode.

Previous behaviour:

# Before
* First sentence. Second sentence.

# After (enter is pressed just before 'S' in insert mode)
* First sentence. 
Second sentence.

New behaviour:

# Before
* First sentence. Second sentence.

# After (enter is pressed just before 'S' in insert mode)
* First sentence. 
* Second sentence.

[cdalizadeh]

This line was causing tests to fail. Happy to add it back if needed.

[harshad1]

This appears to be failing after the most recent changes. Could you take a look? I can merge once tests pass