`indent-and-new-item` should effect blockquotes

[MyriaCore]

markdown-indent-on-enter's indent-and-new-item should be applied to quotes too. While they're not lists, per-se, they functionally are when people are doing transcribing work.

ATM, the current behavior (at least on my emacs) is that when the user presses the enter key while typing out a quote, they start on a new line.

[MyriaCore]

Looking at the code, it looks like a change would need to be made to markdown-regex-list? Not sure if this would break other stuff though.

https://github.com/jrblevin/markdown-mode/blob/cf6403186119cd3d25adc702845f969071060e20/markdown-mode.el#L751-L761

[MyriaCore]

list-marker is defined here:

https://github.com/jrblevin/markdown-mode/blob/cf6403186119cd3d25adc702845f969071060e20/markdown-mode.el#L601-L613

So maybe even just modifying this would work? Something like:

      (list-marker . ,(rx (or (and (one-or-more (any "0-9#")) ".")
                              (any ">*+:-"))))