Improve rendering to a file

[alvarolopez]

We need to improve the handling of output files.

Currently, only writing the output to a single file is supported. However, a reporter may render more than one file (for example several PNG files). We need to handle this and I foresee two possible solutions:

• Specify just the prefix of the output file, and then generate a suffix for each of the generated files (i.e. report-001.png, report-002.png).
• Specify a directory for the outputs, and generate there the files, either secuencially (001, 002) or by date.