pretty completition for pointed files

[Andrew Svetlov]

If user specifies file system shold follow completition rules as shells does.
For examle 'blog html article1'.
Context is: some file with .rst extension.
If current folder structure is:
- article1.html
- article1.inner.html
- article1.rst
- article2.rst
finder should return article1.rst (fullname probably).
The same for 'article1.'

If specified file name allows multiple exnedings (for example, 'article' in our case) finder should raise exception with text like:
 'file' parameter 'article' is ambigous.