SSG build on page change

[ragavendra]

Goals

1. SSG pages should not build pages during build time if it has already generated the same page in the earlier build(s).
2. SSG should generate a new page if the page is different from the last one it built during the last build time. This can save a lot of duplicate(s) build time.

Non-Goals

Background

This feature can save a lot of build time trying to build the already built pages again especially when there is a lots of SSG pages.

Proposal

Not sure on this part, but nice if this setting is the default build behaviour.