Add OpenWhisk sequence support

[develpr]

Issue #93 involves adding support for OpenWhisk. There may be a number of issues changes required to fully support OpenWhisk. This is a bare minimum I believe, because without this change an exception will always be thrown for OpenWhisk sequences if they do not contain a handler.

In this pull request I filter out functions that have a sequence defined for them only if they also do not have a handler supplied. We essentially "skip" over the TypeScript plugin handling logic in this case and allow other plugins / processes to act accordingly for the particular function.

I also updated the ServerlessOptions model to allow for an optional sequence to be defined on the class.

[develpr]

Any chance this would be reviewed at some point?