feat(plugins): add getLastRelease hook

The code to determine the last-release is now in its own repository: https://github.com/semantic-release/last-release-npm
One can overwrite that behavior by defining a "getLastRelease" plugin. This way
one can decouple semantic-release from npm, e.g. by implementing a git-tag
based version. This is already worked on: https://github.com/semantic-release/last-release-git-tag

Closes #56

test/specs/last-release.js

@@ -1,79 +0,0 @@
-const { defaults } = require('lodash')
-const test = require('tap').test
-
-require('../mocks/registry')
-const lastRelease = require('../../dist/lib/last-release')
-
-const npm = {
-  registry: 'http://registry.npmjs.org/',
-  tag: 'latest'
-}
-
-test('last release from registry', (t) => {
-  t.plan(5)
-
-  t.test('get release from package name', (tt) => {
-    lastRelease({
-      pkg: {name: 'available'},
-      npm
-    }, (err, release) => {
-      tt.error(err)
-      tt.is(release.version, '1.33.7', 'version')
-      tt.is(release.gitHead, 'HEAD', 'gitHead')
-      tt.is(release.tag, 'latest', 'dist-tag')
-
-      tt.end()
-    })
-  })
-
-  t.test('get release from a tagged package\'s name', (tt) => {
-    lastRelease({
-      pkg: {name: 'tagged'},
-      npm: defaults({tag: 'foo'}, npm)
-    }, (err, release) => {
-      tt.error(err)
-      tt.is(release.version, '0.8.15', 'version')
-      tt.is(release.gitHead, 'bar', 'gitHead')
-      tt.is(release.tag, 'foo', 'dist-tag')
-
-      tt.end()
-    })
-  })
-
-  t.test('get error from an untagged package\'s name', (tt) => {
-    lastRelease({
-      pkg: {name: 'untagged'},
-      npm: defaults({tag: 'bar'}, npm)
-    }, (err) => {
-      tt.is(err.code, 'ENODISTTAG', 'error')
-
-      tt.end()
-    })
-  })
-
-  t.test('get release from scoped package name', (tt) => {
-    lastRelease({
-      pkg: {name: '@scoped/available'},
-      npm
-    }, (err, release) => {
-      tt.error(err)
-      tt.is(release.version, '1.33.7', 'version')
-      tt.is(release.gitHead, 'HEAD', 'gitHead')
-      tt.is(release.tag, 'latest', 'dist-tag')
-
-      tt.end()
-    })
-  })
-
-  t.test('get nothing from not yet published package name', (tt) => {
-    lastRelease({
-      pkg: {name: 'unavailable'},
-      npm
-    }, (err, release) => {
-      tt.error(err)
-      tt.is(release.version, undefined, 'no version')
-
-      tt.end()
-    })
-  })
-})

test/specs/pre.js

@@ -6,9 +6,16 @@ const pre = proxyquire('../../dist/pre', {
   'child_process': require('../mocks/child-process')
 })
 
+const versions = {
+  available: '1.0.0'
+}
+
 const plugins = {
   verifyRelease: (release, cb) => cb(null, release),
-  analyzeCommits: (commits, cb) => cb(null, 'major')
+  analyzeCommits: (commits, cb) => cb(null, 'major'),
+  getLastRelease: ({ pkg }, cb) => {
+    cb(null, { version: versions[pkg.name] || null, gitHead: 'HEAD' })
+  }
 }
 
 const npm = {