change metalsmith dependency to peerDependency

[depuits]

metalsmith.match was added since version ^2.4.1. Because of a package dependency to version ^2.3.0 this creates error in plugins using the match method. As pointed out in issue metalsmith/sass#4 (comment) it would be better to add the metalsmith dependency as peerDependency to avoid these problem.

[fidian]

I originally authored this plugin and I no longer have access to this organization. Ideally, they will maintain the plugin but I would wager that isn't going to happen.

If you like, I have addressed this issue with @fidian/metalsmith-sugar (GitHub). 1.2.2 was released moments ago to fix this.