Add footer

Options:
  --footer=STR             String to print as footer
  --footer-border[=STYLE]  Draw border around the footer section
                           [rounded|sharp|bold|block|thinblock|double|horizontal|vertical|
                            top|bottom|left|right|line|none] (default: line)
  --footer-label=LABEL     Label to print on the footer border
  --footer-label-pos=COL   Position of the footer label
                           [POSITIVE_INTEGER: columns from left|
                            NEGATIVE_INTEGER: columns from right][:bottom]
                           (default: 0 or center)

The default border type for footer is 'line', which draws a single
separator between the footer and the list. It changes its position
depending on `--layout`, so you don't have to manually switch between
'top' and 'bottom'

The 'line' style is now supported by other border types as well.
`--list-border` is the only exception.

src/core.go

@@ -39,7 +39,7 @@ func (r revision) compatible(other revision) bool {
 // Run starts fzf
 func Run(opts *Options) (int, error) {
 	if opts.Filter == nil {
-		if opts.Tmux != nil && len(os.Getenv("TMUX")) > 0 && opts.Tmux.index >= opts.Height.index {
+		if opts.useTmux() {
 			return runTmux(os.Args, opts)
 		}