use functional option for cui

[micnncim]

Thanks for a great tool.
I believe that using the functional option pattern makes cui.UI initialization more extensible.
And maybe the arg nil for app.New is not best solution, right?
If you check out a piece of code and feel pleased, please take this in.

ref

• https://github.com/motemen/gore/blob/master/gore.go#L25-L31
• https://github.com/motemen/gore/blob/master/option.go

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d48f2bd). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #174   +/-   ##
=========================================
  Coverage          ?   66.46%           
=========================================
  Files             ?       54           
  Lines             ?     2496           
  Branches          ?        0           
=========================================
  Hits              ?     1659           
  Misses            ?      697           
  Partials          ?      140

[ktr0731]

Thank you for submitting a PR, LGTM 👍