Add registers

[kyoheiu]

Implement #204

:reg - list the contents of register a and the default register
"ay (visual mode) - yank selected files into register a
"ad (visual mode) - delete selected files and yank into register a
"Ay (visual mode) - yank selected files and append into register a
"Ad (visual mode) - delete selected files, yank and append into register a
"ayy - yank selected file into register a
"add - delete and yank select file into register a
"Ayy - yank selected file and append into register a
"Add - delete selected file, yank and append into register a
"ap - put contents of register a into current directory

Also, refactored unpack.