Enable partial sharing

It is currently not possible to disable the sharing on a specific
site, say google plus or facebook.

This allows a configuration item switch to tweak this behavior.

exampleSite/config.toml

@@ -25,6 +25,9 @@ faviconfile = "img/logo.png"
 highlightjs = true
 progressively = true
 share = true
+# Share to googleplus, twitter, fb, linkedin by default
+# disable by setting share_on_<social network> = false
+# example: share_on_googleplus = false
 latestpostcount = 5
 github = "example"
 email = "you@example.com"