Use go templates for css and html output

[adrinux]

Currently palegen.html and styles.css are static files. These should be reworked into go templates to make maintenance less tedious. Also need to add styles.css code inline in palegen.html.

Could add a command line flag to only output palegen.html when requested, dependent on issue #2

[adrinux]

Use SVG instead of HTML? Would make embedding in other documents easier.

[adrinux]

Should output css to colors.css for use in projects but also output CSS inline in the palegen.html file so it can be dropped into projects as a standalone palette page.