Allow customizing the file preview command (#156)

To allow users to implement custom file previews (great ideas include previewing images or showing hexdumps), the command used to preview files is now configurable. To change it, set the variable fzf_preview_file_cmd.

Huge thanks to @neersighted who not only came up with the idea, but implemented it within a couple short hours!

Bug fixes

• [Search git log] improve compatibility with older git versions (#151 - @kidonng )
• [Search directory] fix preview for files with spaces (again) (#152 - with help from @kidonng )