perf hists browser: Implement printing snapshots to files

To avoid having to resort to --stdio, that expands everything, instead
allow the user to go on expanding the relevant callchains and then press
'P' to print that view.

As the hists browser is used for both static (report) and dynamic (top)
views, it prints to a 'perf.hists.N' sequence, i.e. multiple snapshots
can be taken in report and top.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-wr9xx4ba0utrynu5j6wotd79@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>