support vimwiki style links.

[jahagirdar]

In vimwiki

The code [Sales](Sales) Points to the file 'Sales.md'
And [DebugInfrastructure/index](DebugInfrastructure/index) points to the file DebugInfrastructure/index.md

Markserv throws an error of:

ENOENT: no such file or directory, stat '{MarkdownDir}/Sales'

for these links as it does not automatically add an '.md' extension while searching for the file.

[thorlucas]

I’d love to see this as well.

[slarrain]

We very much need this.