Custom extensions

This pull request introduces support for several new features.

1. Add editor options under **Tools &rarr; Options... &rarr; Text Editor &rarr; Markdown** (line numbers, tabs, and indentation)
2. Support custom file extensions for Markdown in **Tools &rarr; Options... &rarr; Text Editor &rarr; File Extension**
3. Support opening a file in the Markdown editor using the **Open With** command on the Open File dialog