Adds publish/subscribe pattern to allow plugins and projects to hook into events (#55)

Also adds CLI to ease development.

packages/medusa-test-utils/yarn.lock

@@ -861,6 +861,13 @@
     levenary "^1.1.0"
     semver "^5.5.0"
 
+"@babel/runtime@^7.9.6":
+  version "7.9.6"
+  resolved "https://registry.yarnpkg.com/@babel/runtime/-/runtime-7.9.6.tgz#a9102eb5cadedf3f31d08a9ecf294af7827ea29f"
+  integrity sha512-64AF1xY3OAkFHqOb9s4jpgk1Mm5vDZ4L3acHvAml+53nO1XbXLuDodsVpO4OIUsmemlUHMxNdYMNJmsvOwLrvQ==
+  dependencies:
+    regenerator-runtime "^0.13.4"
+
 "@babel/template@^7.3.3", "@babel/template@^7.7.4", "@babel/template@^7.8.6":
   version "7.8.6"
   resolved "https://registry.yarnpkg.com/@babel/template/-/template-7.8.6.tgz#86b22af15f828dfb086474f964dcc3e39c43ce2b"
@@ -4064,6 +4071,11 @@ regenerate@^1.4.0:
   resolved "https://registry.yarnpkg.com/regenerate/-/regenerate-1.4.0.tgz#4a856ec4b56e4077c557589cae85e7a4c8869a11"
   integrity sha512-1G6jJVDWrt0rK99kBjvEtziZNCICAuvIPkSiUFIQxVP06RCVpq3dmDo2oi6ABpYaDYaTRr67BEhL8r1wgEZZKg==
 
+regenerator-runtime@^0.13.4:
+  version "0.13.5"
+  resolved "https://registry.yarnpkg.com/regenerator-runtime/-/regenerator-runtime-0.13.5.tgz#d878a1d094b4306d10b9096484b33ebd55e26697"
+  integrity sha512-ZS5w8CpKFinUzOwW3c83oPeVXoNsrLsaCoLtJvAClH135j/R77RuymhiSErhm2lKcwSCIpmvIWSbDkIfAqKQlA==
+
 regenerator-transform@^0.14.0:
   version "0.14.1"
   resolved "https://registry.yarnpkg.com/regenerator-transform/-/regenerator-transform-0.14.1.tgz#3b2fce4e1ab7732c08f665dfdb314749c7ddd2fb"