Add meta-viewport tag only in html5 mode

See #114
Move theme support check to thematic_meta_viewport() function

See #114
Add the initial-scale=1 to meta viewport tag

See #114
Clarify code comment for meta viewport theme_support in theme setup
Merge pull request #116 from ThematicTheme/feature/meta-viewport-initial-scale

Feature/meta viewport initial scale
Closes #114