There is two ways to capture output of command in shell scripts:

 * `command args`
 * $(command args)

Both are supported by POSIX and all shells in wide use (except of
Solaris /bin/sh) but second one is simplier to read, especially in case of
nesting.

Surfraw project already uses both form, so changing all code to use
single, more modern and readable form, do not cause loss of portability.

Since currently codebase already uses both forms, POSIX-compatible shell
is already required. Also, `HACKING' file is modified to suggest new
form of command output capture.

For more information about two forms of output capture, see
https://stackoverflow.com/questions/9449778