google-codelab: skip non-parseable duration
google-codelab: skip non-parseable duration

Some steps may not specify duration. This resulted in the total
codelab time to be NaN.

So, only count duration values which are greater than 0.

Fixes #40

googlecodelabs/codelab-elements,googlecodelabs/codelab-elements,googlecodelabs/codelab-elements,googlecodelabs/codelab-elements,googlecodelabs/codelab-elements