Exclude devices from disk usage statistics

[owahlberg]

Added a simple option to exclude devices or partitions based on a regular expression.
I use this to exclude very specific devices without complicating the default devices regex.

[coveralls]

Coverage remained the same at 59.217% when pulling ddfc784 on owahlberg:exclude_devices into e044ec4 on python-diamond:master.

[shortdudey123]

Same as #444

[shortdudey123]

Can you update the tests and doc?

[shortdudey123]

This is a subset of the commits in #483