[pipelines] Expose logs generated by build tools and other stuff

Hello,

BB Pipelines is a great tool for CI automation.
However, when things go wrong it is a nightmare to debug, specially when the problem only exist on pipelines and it is impossible to reproduce locally.

Most build tools generate logs with detailed information about what they do and errors or problems they encounter. NPM for example, when something fails gives you the following message (copied from a failed pipeline)

npm ERR! A complete log of this run can be found in:
npm ERR!     /root/.npm/_logs/2018-10-09T07_52_02_345Z-debug.log

It will be extremely valuable to read those logs and see the actual source of the problem with great detail.

I understand that supporting all the build tools on the market is an impossible task, so maybe you could accept this information on bitbucket-pipelines.yml file.

Thanks and regards.