Fix done.txt naming bugs

[isti03]

Two unreported done.txt naming bugs fixed:

1. .Todo.txt files would use .done.txt for the done filename

2. When a user had the ignorecase option set, the done.txt file was always named Done.txt, because the == operator ignores the case when this option is set.

The bugs were introduced in PR https://github.com/freitass/todo.txt-vim/pull/73

Further readings on the topic:

• Comparisons / Learn Vimscript the Hard Way
• Vim documentation: eval

[isti03]

I should do more rigourous testing before pushing...

Now it should be all good