hype preview depends on tree #114

$ hype preview -theme github -f hype.md 
hype version dev (commit: none, built: unknown)
$ hype
------
*github.com/gopherguides/hype/cmd/hype/cli.App

Available Commands:
  Command   Description
  -------   -----------
  blog      static blog generator commands (init, build, new)
  export    export the document to a different format (markdown,json,html,etc...)
  marked    outputs for the Marked app (https://marked2app.com/)
  preview   live preview server with file watching and auto-reload
  slides    outputs slide format for presentation
  validate  validate document integrity without full export
  version   print version information

Error: initial build failed: execute error: filepath: /home/dolanor/src/hype/hype.md
document: Hype
execute error: filepath: /home/dolanor/src/hype/hype.md
cmd: $ tree ./docs
exit: -1
cmd error: exec: "tree": executable file not found in $PATH

Error: initial build failed: execute error: filepath: /home/dolanor/src/hype/hype.md
document: Hype
execute error: filepath: /home/dolanor/src/hype/hype.md
cmd: $ tree ./docs
exit: -1
cmd error: exec: "tree": executable file not found in $PATH