Add config option for brackets pattern

[Alexander Porsin]

For those who use the Twig, construction <-- {env}:{file} --> can be replaced by {# {env}:{file} #} for cleaning output html, and default for other users.
Maybe add to gassetic.yaml parameter with default value ['<--', '-->']?
replace_brackets: ['{#', '#}']

[Roman Schejbal]

Closing this since we are going in a slightly different direction. See #62