Support loading scripts from configmaps

[julienduchesne]

K6 supports running tests from a directory structure, using imports. This could be supported by loading common config maps (one or many)

The benefits of this are:

• Reusing the same tests for multiple deployments
• Cleaner, more structured test definitions

[julienduchesne]

As mentioned here: #15 (comment). Another solution to use multiple files in a single script is to transpile them with rollup.js