Standard exports

The procfs package can be used to query the pseudo-filesystem commonly
mounted under /proc. It currently provides information from /proc/stat,
/proc/$pid/stat, /proc/$pid/fd and /proc/$pid/limits.

This change adds two new collectors to the prometheus package which
export metrics about a given or the current process.

* ProcessCollector exports metrics about cpu time, vss, rss, fd usage as
  well as the start time of a given process.
* GoCollector exports currently only the number of active goroutines.

Both collectors are active by default.

Background: https://docs.google.com/document/d/1Q0MXWdwp1mdXCzNRak6bW5LLVylVRXhdi7_21Sg15xQ/edit?usp=sharing
As above:

    ch <- c.cpuTotal

etc. below.

Yet another possibility, which is more compact, but also more low-level (as the suggestion above, this is all optional and whatever you prefer):

Instead of the metrics in the processCollector, just have the Desc's there.
Create them in the constructor with `NewDesc`, e.g.

    startTime: NewDesc(
        namespace+"_process_start_time_seconds",
        "Start time of the process since unix epoch in seconds.",
        nil, nil,
    )

Then, in `Describe()` just write `ch <- c.startTime`.

Here, in the collect part, instead of `Set` and then `Collect`, it's only one statement:

    ch <- MustNewConstMetric(c.startTime, GaugeValue, stats.CPUTime())