Add basic support for JSON output with option '-j'

* strace.c(init, usage, tprints, tprintf): Add support for parse '-j' option,
Modify tprint* use the hook framework.
* strace.1: Add description for '-j'
* output.c: New file. Provide the output hook framework
* Makefile.am: Add the new file output.c to strace_SOURCES
Modify sys_read()/sys_write() to support JSON output

* io.c(sys_read, sys_write): Use the hook framework to
support JSON output.
Simple test for JSON output

Add some very simple test for JSON output in test directory.