Optional data cleaning step

[chfleming]

As a long-term goal, it would be nice to have an optional step where users could import their data and clean/edit it before as.telemetry(). Maybe something like Open Refine could serve this purpose without too much hassle.

Typically people use spreadsheet programs, like Microsoft Excel, but we try to steer them away from that because (1) its dangerous to edit your raw data and (2) Microsoft Excel automatically mangles timestamps in a way that is not consistent across their own operating systems.