Add a concurrency parameter to specify the max number of builds at the same time

[javiortizmol]

Add a concurrency parameter to specify the max number of builds at the same time like the webpack plugin has.

https://www.serverless.com/plugins/serverless-webpack

[pgrzesik]

Hey @javiortizmol - thanks for your proposal. We'd be happy to consider a community contribution for that feature if anyone would be interested 💯