Support sigrok capture file format

[mithro]

sigrok (https://sigrok.org/wiki/Main_Page) is a very popular tool for interfacing with logic analyzers and doing protocol decoding. There are thus a lot of waveform captures out there in this format and a bunch of tools like LiteX support outputting captures in the format.

I believe there is a bunch of information at https://sigrok.org/wiki/Input_output_formats and https://sigrok.org/wiki/File_format:Sigrok/v2