support submit-on-valid-parse

[JeanMertz]

This allows CustomType prompts to auto-submit when the input is valid. Specifically, this makes it possible to have prompts similar to Git's:

$ git add -p
... <snip> ...

(1/2) Stage this hunk [y,n,q,a,d,j,J,g,/,e,p,?]? ?
y - stage this hunk
n - do not stage this hunk
q - quit; do not stage this hunk or any of the remaining ones
a - stage this hunk and all later hunks in the file
d - do not stage this hunk or any of the later hunks in the file
j - leave this hunk undecided, see next undecided hunk
J - leave this hunk undecided, see next hunk
g - select a hunk to go to
/ - search for a hunk matching the given regex
e - manually edit the current hunk
p - print the current hunk, 'P' to use the pager
? - print help
(1/2) Stage this hunk [y,n,q,a,d,j,J,g,/,e,p,?]? q

Which don't require the return key to be pressed when a valid option is provided.

[JeanMertz]

I just noticed #317 which does something similar, but for the Confirm prompt.

I took some of their code, and ported it to this more generic implementation, so now the Confirm prompt also has a confirm_on_input option.