Filename prefixed image directory name

[Hritik14]

This commit tries to work on #49
An option to put all the images of current markdown file in a
directory with current filename without the extension prefixed to
mdip_imgdirwith an underscore in between.
This is supposed to help organize images related to multiple
markdown documents in the same directory.

For more customization, use expand() inside mdip_imgdir
Eg:

" The following will save all the images in the directory
../all_images/currentfilename_without_extension
let g:mdip_imgdir = '../all_images' . expand('%:r')

[Hritik14]

@ysl2 Thanks for the approval. Will this be merged ?

[ferrine]

Your code assumes you work with only one file. Opening another file within the session will not use a different name for prefix

[ferrine]

I think this PR is covered by #61, I'll reopen it on your notice if you want to continue