Add "pretty" log variants for more flexible log formatting.

Variant views are triggered by passing the 'pretty=FORMAT' URL param, such
as 'pretty=full' or 'pretty=oneline'.

Additional variants or overrides can be declared in gitiles.config with:
  [logFormat "oneline"]
    variant = myOnelineVariantOverride
  [logFormat "special"]
    variant = myCompletelyCustomVariant

Change-Id: If30554995062110bfbf22ee8760db8303d50b69c