Improve UX of conceallevel UI warning

[larrybotha]

With the current implementation, the warning is always shown under the following conditions:

• conceallevel is something other than 1 or 2 globally
• the user sets conceallevel = 1 | 2 locally for markdown files, but opens Neovim with a buffer that is not markdown

This can be cumbersome if:

• the user's config runs obsidian.setup in a non-Obsidian repo and conceallevel is not 1 or 2
• the user sets conceallevel for markdown files only, but opens Neovim in an Obsidian repo on a non-markdown file

This PR addresses these scenarios by:

• using an autocommand to assert conceallevel on BufEnter only for markdown files
• only runs the autocommand once per call of obsidian.setup

ref #286

EDIT: add friction points, add ref to issue where warning was added

[marcofeltmann]

Is there any way to deactivate that warning in total?
I know that the rendering fails and I want it to show the square brackets instead of the ugly icons.

I totally understand the warning itself is a pretty good idea, but I'd love something like

require'obsidian'.setup({'conceallevelwarning' = 'skip'})

[marcofeltmann]

TBH but that might be a bug (in the plugin or in my brain)

If I have the following setup:

1. conceallevel is 0 globally
2. I have conceallevel = 2 locally for markdown files and opens Neovim with a buffer that is markdown

that warning appears directly after startup.

If I set conceallevel = 1 globally and conceallevel = 2 for markdown files there is no warning.

(Oh, yeah, commit e9a8fd0 it is on my machine, included via nvim pack, if that might be interesting)

[epwalsh]

Thanks @larrybotha, LGTM

[epwalsh]

@marcofeltmann you can avoid the warning by disabling our additional syntax stuff. Does that work for you?