Add command-line editing support for interactive pprof

This implements basic command-line editing, using
github.com/chzyer/readline. This will not affect the golang
distribution as it uses its own pprof driver, with its own
driver.

Only tested on Linux.
TODO: Implement auto-completion and persistent history.

- Remove dead code, unnecessary conversions
- Consistently use a single space after period

Also update the build instructions to use go get