Always add the default theme

Previously, Themr would check to see if the default theme file existed before adding it to the list of themes. However, in Sublime Text 3, the default package files (including the default theme) are contained in a zip file in the application directory. So Themr would never find the default theme file in ST3, and it would never be added to the list.

Since it's a pretty safe bet the default theme always exists (especially in ST3, where it's contained in the zip file,) Themr now always adds it to the list without checking it first.