Update metalsmith to version 2.1.0 🚀

[greenkeeperio-bot]

Hello :wave:

:rocket::rocket::rocket:

metalsmith just published its new version 2.1.0, which is not covered by your current version range.

If this pull request passes your tests you can publish your software with the latest version of metalsmith – otherwise use this branch to work on adaptions and fixes.

Happy fixing and merging :palm_tree:

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The new version differs by 29 commits .

• e1d6e93 2.1.0
• f3720ef Merge pull request #171 from Ajedi32/frontmatter-cli-config
• f5fd713 Add support for frontmatter config to Metalsmith CLI
• fc87e74 Merge pull request #168 from segmentio/add/slackin
• 836f60a added slackin
• d78a204 Merge pull request #167 from Ajedi32/node-4
• 075fabe Add Node 4.0 to Travis build
• bfa0b08 Merge pull request #163 from woodyrew/patch-1
• db698ea Merge pull request #162 from kmiyashiro/patch-1
• e9c9a14 Merge pull request #150 from arve0/patch-1
• 2a1ba3b Merge pull request #160 from Ajedi32/fix-plugin-array-test
• d47fed5 Added troubleshooting for changes to 2.0.
• b243ae1 Fix to incorrect ignore parameter in README
• 10efd3b Fix plugin array test
• c0f5009 .use calls function metalsmith instance, not metadata

There are 29 commits in total. See the full diff.

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