add go.mod for example directory

[ktr0731]

In the latest Go, examples in _example can't run directly because these examples depend on some untracked libraries.

$ go run ./_example/cli
_example/cli/main.go:11:2: no required module provides package github.com/mattn/go-isatty; to add it:
        go get github.com/mattn/go-isatty
_example/cli/main.go:12:2: no required module provides package github.com/spf13/pflag; to add it:
        go get github.com/spf13/pflag

To avoid this, I added another go.mod to example directory (renamed from _example).

[codecov]

Codecov Report

Merging #131 (376751b) into master (35bbbaa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #131   +/-   ##
=======================================
  Coverage   85.73%   85.73%           
=======================================
  Files           5        5           
  Lines         764      764           
=======================================
  Hits          655      655           
  Misses         95       95           
  Partials       14       14