fix: resolve custom theme from global cli (vuejs#392)

Author: meteorlxy

lib/prepare.js

@@ -136,7 +136,12 @@ async function resolveOptions (sourceDir) {
     if (siteConfig.theme) {
       // use external theme
       try {
-        themeLayoutPath = require.resolve(`vuepress-theme-${siteConfig.theme}/Layout.vue`)
+        themeLayoutPath = require.resolve(`vuepress-theme-${siteConfig.theme}/Layout.vue`, {
+          paths: [
+            path.resolve(__dirname),
+            path.resolve(sourceDir)
+          ]
+        })
         themePath = path.dirname(themeLayoutPath)
       } catch (e) {
         throw new Error(`[vuepress] Failed to load custom theme "${