use strings.ToValidUTF8 to validate string

[nokute78]

ref #19

The patch replaces invalid utf8 string with whitespace.
e.g.
hello\x80world -> hello world

I just modified only plain*.go. Could you review it ?
If the review is ok, I will modify json*.go and logfmt*.go.

[ymmt2005]

@nokute78
Thank you for the PR!

I'd like to use utf8.RuneError character instead of the white space.
Could you update the PR?

[nokute78]

@ymmt2005 Thank you for quick reviewing.

I updated the PR.

[ymmt2005]

Looks good!

Please update json and logfmt the samely.

[nokute78]

@ymmt2005 Thank you for reviewing.
I modified json/logfmt files.

Note: json_test.go already tests invalid utf8 string, so I didn't modify it.

[ymmt2005]

LGTM.

@Hsn723 Could you take another look?

[Hsn723]

@nokute78
Please add the go 1.13 directive to go.mod as strings.ToValidUTF8 can only be used starting go 1.13. Other than that, looks good.

@ymmt2005
After merging these changes, I'll update the Readme to indicate that go 1.13 or greater is required and cut a new v1.6.0 release.

[nokute78]

@Hsn723 @ymmt2005 Thank you for reviewing.
I added go directive.

[Hsn723]

LGTM
@nokute78 Thank you for your contribution.