Dup paths on write
Copy mime type changes on write
Copy engines on write
Extract configuration module
Extend all off configuration
Centralize configuration copying
Whitespace
No need for parent environment ref
Add mutate config helper
Use mutating helpers on processors
Use return value from mutate block
Merge branch 'options-copy-on-write'
Merge branch 'master' into content-encoding

Conflicts:
	lib/sprockets.rb
	lib/sprockets/cached_environment.rb
	lib/sprockets/environment.rb