support vimwiki style links.

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.