expand/improve default scripts

[Ckath]

the default open and preview scripts are merely ported and stripped down versions of rangers scripts, as I don't personally use a lot of programs I'm not sure how sensible these defaults are.

the following open/preview improvements are always welcome either as comment, separate issue or pr:

• implementations of unsupported filetypes
• more sensible order of programs or filetypes
• more alternative programs to handle filetypes
• any other arguable improvements

[Ckath]

examples of such small edits to the scripts:

• add some support for flash files: 7cf171e cc2417e
• stop warnings from overwriting image previews: 829ee53