Print external commands that ruby-build executes

ruby-build now prints the full invocation of (almost) every external
command that it runs. Typically that is something like:

    -> ./configure --prefix=/path/to/ruby
    -> make -j 2
    -> make install

All output of these commands still goes to the log file by default.

This changes the behavior of `--verbose` mode to simply redirect all
command output to the stdout & stderr of the parent process instead of
writing to and then tailing the log file. This allows implementations of
commands like `./configure` or `make` to detect a terminal and output
color.