Add support for libsigrok

[maxregan]

Consider adding support for libsigrok and libsigrokdecode. From their docs:

"The sigrok project aims at creating a portable, cross-platform, Free/Libre/Open-Source signal analysis software suite that supports various device types (such as logic analyzers, oscilloscopes, multimeters, and more)."

This would add nanaimo support for a large range of devices, making it more of a "batteries-included" solution. It works for a variety of devices- including Saleae logic analyzers (without running Saleae's Logic software) and variety of common (and uncommon!) protocols for embedded devices.

Potential issues of note, both of which are true for libsigrok and libsigrokdecode:

• Each has two components: a C library which is found in most Linux distributions, and separate Python bindings, which are not available provided by pypi or major OS distributions that I can find
• Each is distributed under LGPL v3, as opposed to nanaimo's more permissive MIT license.

Thanks!

[thirtytwobits]

neat! LGPL v3 might be a deal-breaker however.