File extensions should not be changed on save

MacDown should not change the file extension of a file when it is saved. This is very frustrating when working with files that have an 'outer' extension as well as the normal .md extension.

To reproduce:

1. Open a file that has an extension that is not .md (i.e. my_file.md.erb)
2. Save the file
3. Observe the file extension changing

A possible solution is to provide an option to disable this, or to only change the file extension if there is no .md anywhere in the title, so that it will not affect .erb files.