Add option to output tiller logs to stderr

[rdpa]

Right now there are options to output tiller logs to a file. I would like an option to output tiller logs to stderr after the build for CI log purposes.

[luisdavim]

#66 should make it easy to implement sending the output to stderr.

[andrewnazarov]

#66 has introduced some issues: #67

[luisdavim]

I think that if you just set HELM_TILLER_LOGS_DIR to /dev/fd/2 you'll get the tiller logs in stderr.

[luisdavim]

I've opened #69 to address the issues caused by #66 sorry about that.