Add config option to set default mode on markdown widget

[cdeath]

Is your feature request related to a problem? Please describe.

I want to be able to set the markdown widget in markdown mode by default and keep rich text as optional.

Describe the solution you'd like

a solution could be when defining modes for the markdown widget on config.yml to honor the array order:
(e.g. modes: ["raw", "rich_text"], should set markdown by default`)

Additional context

default:

with modes: ["raw", "rich_text"]:

[erezrokah]

Hi @cdeath, please note that Rich Text/Markdown selection is "sticky" - the preference is persisted in local storage.

[cdeath]

@erezrokah even better... but most will never touch the preference and i want my audience to use markdown primarily. if one team member wants to use Rich Text it's up to him (and to take the blame for breaking stuff).

for more context: we're using kramdown extensions in some places which look weird in Rich Text mode and might break but i don't want to exclude Rich Text... and about the team, if they don't know markdown or kramdown they can go RTFM lol