add outline-minor-mode with markdown headings

[cosmicexplorer]

outline-minor-mode allows traversing and folding headings, just like the outline-mode major mode. The keybindings don't conflict with any bindings already in markdown-mode, and allow folding just like org-mode. I couldn't think of what tests would be appropriate since this just adds the functionality outline-minor-mode already does, but I can definitely add some as required.

To play with it, just open a markdown file and try C-c @ C-c to hide, and C-c @ C-e to show a heading. There are more complex controls too, all the same as in outline-mode.

[syohex]

outline-regexp is already set here

[cosmicexplorer]

Oops, thanks. Removed.

[jrblevin]

markdown-mode already has section folding functions which are robust to certain cases that cause outline-minor-mode to fail. Since it is based on a regular expression, it cannot distinguish between hash marks that are comments in code blocks and those that are actual ATX headings, so we run into issues like #33 and #75.

Is the main goal to use the well-known keybindings? If so, we can just add additional bindings that use the custom functions (markdown-show-subtree, markdown-hide-subtree, etc.).

[cosmicexplorer]

Sorry, I didn't realize this had already been considered. Was looking for functions with the word "fold" in them and didn't find the ones already there. I'll look more thoroughly next time.

[cosmicexplorer]

Although, what is the purpose of setting outline-regexp if outline-minor-mode isn't turned on? Is it so that people can add outline-minor-mode or something themselves?

EDIT: sorry about reopening this, misclicked.

[jrblevin]

I think that was the original idea (allowing people to turn on outline-minor-mode if/when they wanted). At this point, I'd say we only use the name outline-regexp for historical reasons. I wouldn't really recommend using outline-minor-mode over the functions we have now (for both movement and folding).

[cosmicexplorer]

Ok, makes sense.