Solve #68 and #69. Add the names of metapackages that have the specific programs for file previews as dependencies. 

This helps people to easily discover and install programs used for previewing files of various types.

Change 'Meta-Package' to 'Meta-Packages'.

Remove the third column called 'Meta-Packages'. The metapackages will be referenced in the second column through a link to the websites pertaining to the programs.

Add links to programs listed for file previews in README.md. Most links are copied from Arch Linux website from package descriptions and checked reliably enough.