feat: add fallbackTag functionality and map "next" to "latest"

You can now map dist-tags onto "fallbackTags". E.g. if you want a
development channel like "next" and no version is currently published as "next"
semantic-release will fallback to "latest" to determine the new "next" version.
This is now the default for "next" -> "latest", but you can specify your own
mappings by adding "fallbackTags" to your `package.json`'s "release" field.

BREAKING CHANGE: In cases where you pushed a new commit with a "dist-tag"
configuration of "next" and no version was previously published as "next",
semanitc-release failed and did not publish a new version. It will now
automatically fall back to to "latest".

package.json

@@ -24,7 +24,7 @@
     "@semantic-release/commit-analyzer": "^2.0.0",
     "@semantic-release/condition-travis": "^4.0.0",
     "@semantic-release/error": "^1.0.0",
-    "@semantic-release/last-release-npm": "^1.1.1",
+    "@semantic-release/last-release-npm": "^1.2.0",
     "@semantic-release/release-notes-generator": "^2.0.0",
     "git-head": "^1.2.1",
     "github": "^0.2.4",

src/index.js

@@ -24,6 +24,9 @@ const options = _.defaults(
   pkg.release,
   {
     branch: 'master',
+    fallbackTags: {
+      next: 'latest'
+    },
     debug: !env.CI,
     githubToken: env.GH_TOKEN || env.GITHUB_TOKEN,
     githubUrl: env.GH_URL