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
This could remove too many ()s, for example:

echo '#!/bin/cat /proc/self/stat' > '('
chmod 755 \(
./\(
10502 (() R 10336 10502 10336 34822 10502 4202496 217 0 0 0 0 0 0 0 20 0 1 0 274309049 7352320 153 18446744073709551615 4194304 4237340 140735267735968 140735267735968 140630052589312 0 0 0 0 0 0 0 17 5 0 0 0 0 0 6336032 6337168 14458880 140735267737687 140735267737716 140735267737716 140735267741684 0
#!/bin/cat /proc/self/stat