Adds config option to use ENTER to send message

[JasonWeill]

Fixes #86.

Adds a config option to swap the uses of Shift+Enter and Enter: the user can choose the default of "Enter sends message" or the alternative of "Shift+Enter sends message":

In the default case, the caption says "Press Shift+Enter to add a new line":

In the alternative case, the caption below the chat box is minimally modified from what you'd see today, "Press Shift+Enter to send message":

Modifies documentation to mention the new default and that users can change the keybindings in the settings pane.

[dlqqq]

Thank you for this work! In addition to the small comment below, I do have one high-level question that I would like to raise: should we change the default from Shift + Enter to Enter? I recall earlier discussion where we had been planning to do this for some time, since chat UIs typically use Enter to send, in contrast to ours.

Made changes requested

[ellisonbg]

I am fine changing the default to Enter.

[JasonWeill]

Thank you for this work! In addition to the small comment below, I do have one high-level question that I would like to raise: should we change the default from Shift + Enter to Enter? I recall earlier discussion where we had been planning to do this for some time, since chat UIs typically use Enter to send, in contrast to ours.

Per conversation with @ellisonbg , I changed the default to Enter.