Merge pull request previm#101 from previm/release-1.8.1

Release 1.8.1

Merge pull request previm#93 from previm/multi-preview

Multi preview

Merge pull request previm#39 from kannokanno/hidden-header

add setting of hidden header

Merge pull request previm#38 from kannokanno/support-mermaid

Support mermaid

update help

README修正

fix previm#1

* previm-function.jsのエンコードを指定

時刻フォーマット修正