add `replace` `only` and `except` plugin

the `metalsmith-replace` can be used to replace string in files' attribute value.

the `metalsmith-only` and `metalsmith-except` can be used to filter files' attribute. 

`metalsmith-only` can filter out the attribute which match the option, and other attributes will be deleted from files.

`metalsmith-except` can filter out the attribute which match the option, and then they will be deleted from files