[Feature Request] Create an org-md-toplevel-hlevel variable to allow users to set the level of top level headings in markdown export

[Rohit Patnaik]

Hello,

I've been exporting several of my org-mode notes to markdown recently, and I've 
been running into an issue where I've had to manually adjust the level of the 
top level headings in my exported notes because the wiki system I'm importing 
the notes to reserves h1 (i.e. #) for page titles, using h2s (i.e. ##) for 
sections within the page itself.

I noticed that ox-html.el has a org-html-toplevel-hlevel variable, which allows 
the user to set the heading level of top-level headings within the exported 
content. Would there be any problems with implementing a similar variable (e.g. 
org-md-toplevel-hlevel) to allow the user to set the heading level of top level 
headings within exported markdown content?

It looks like it would be a small change, and I'd be happy to implement it 
myself, but I thought I'd run it by the list to see if there were any major 
issues that might arise from making this change.

Thanks,
Rohit Patnaik