applet.spi_flash_25c: return subtarget in build() to allow extensions.

applet.spi_flash_25c: identify flashes without command 0x90 support. Such flashes return 0xff as short manufacturer ID, and we might as well display the long manufacturer ID.

Add CONTRIBUTING instructions.

cli: print version if -V/--version option is used.

software: determine own version through versioneer.

applet.spi_master: fix writes to count register. In RECV-COUNT-LSB, this used to overwrite the high bits of count. Fixes #76.

applet.spi_flash_25c: fix progress with read/fast-read to stdout.

protocol.jtag_svf: accept and ignore whitespace in scan data. This is not explicitly described in the SVF specification, but is relied on by real-world tools.

protocol.jtag_svf: Ensure run_count value passed to pulse_tck is integer

applet.jtag: Add state transitions for Pause-DR/Pause-IR