Add basic extension detection.

This add a member to tinydir_file structure : (char *) extension.
This member is filled with the pointer to the last dot find in the
filename.
Change extension detection behavior

If there's no dot in the name, change the extension from NULL to empty
string "".
Change the extension detection

Correct a mistake where adding 1 to NULL.
Change extension pointer if there no extension.

Now the file.extension point to the end of file.name.
Change the empty string location for empty file.extension
Merge pull request #7 from AMDG2/master

Add basic extension detection.