Add insertion property for custom scripts

[Clarko]

Scripts may want to add data at the current insertion point, rather than replace data.

Reason for request: I just wrote a Lorem Ipsum script and realized it can't just insert a lipsum paragraph into an existing string. It can only replace the fullText or replace a (non-zero-length) selection.

Totally invented use cases for insertion:

• get today's date
• roll a die
• flip a coin

[IvanMathy]

Interesting, this is a good idea. I feel like it could work nicely as a function maybe? Something like

state.insert("Something")

In the meantime, you can probably get the same behavior with:

state.text = state.text + "Something"

Not ideal, obviously.

[IvanMathy]

Fixed in #109, look for it in release 1.2.0!