Add option to show link in meta data for editing posts (#278)

Usage

- in site config =>

    Params:
    editPost:
        URL: "https://github.com/<path_to_repo>/content"
        Text: "Suggest Changes" # edit text 
        appendFilePath: true # to append file path to Edit link

- in front-matter vars =>
    ---
    editPost:
        URL: "https://github.com/<path_to_repo>/content"
        Text: "Suggest Changes" # edit text 
        appendFilePath: true # to append file path to Edit link
    ---

- Front-matter vars overrides global ones

layouts/_default/single.html

@@ -16,6 +16,7 @@
     <div class="post-meta">
       {{- partial "post_meta.html" .  -}}
       {{- partial "translation_list.html" . -}}
+      {{- partial "edit_post.html" . -}}
     </div>
     {{- end}}
   </header>