added "label_prefix" to config

when a label_prefix is defined in the config file, it is prepended to the har name.
this enables us to identify and sort hars once captured.

for example, if this was run from a machine in china, we might define the 'label_prefix' like:
label_prefix: china-

this will then create file names like "china-https-www-edx-org-1415891917.59.har" and page refs like "china-https-www-edx-org"
nit: our pattern around here seems to be to always use string formatting rather than concatenation.