in_exec: EXEC input plugin

[enukane]

This patch introduces in_exec plugin which is a subset of "in_exec" in fluentd.

I've tested this in Linux & NetBSD environment as below.
The configuration file is like this.

[EXEC]
    command ls -al
    run_interval 2

(currently supports only "command" and "run_interval" parameters)

Then executes like:

% bin/fluent-bit --config=exec.cfg -i exec -o stdout  
Fluent-Bit v0.1.0
Copyright (C) Treasure Data

[2015/06/05 15:04:06] [ info] starting engine
[0] {"time"=>1433484248, "command"=>"total 72
drwxrwxr-x  9 enukane enukane  4096 Jun  5 15:00 .
drwxrwxr-x 10 enukane enukane  4096 Jun  5 15:00 ..
-rw-rw-r--  1 enukane enukane     0 Jun  5 15:00 .empty
-rw-rw-r--  1 enukane enukane 15276 Jun  5 14:54 CMakeCache.txt
drwxrwxr-x  4 enukane enukane  4096 Jun  5 14:54 CMakeFiles
-rw-rw-r--  1 enukane enukane 11822 Jun  5 14:54 Makefile
drwxrwxr-x  2 enukane enukane  4096 Jun  5 14:54 bin
-rw-rw-r--  1 enukane enukane  2288 Jun  5 14:54 cmake_install.cmake
-rw-rw-r--  1 enukane enukane    46 Jun  5 14:54 exec.cfg
drwxrwxr-x  3 enukane enukane  4096 Jun  5 14:54 include
drwxrwxr-x  5 enukane enukane  4096 Jun  5 14:54 lib
drwxrwxr-x  2 enukane enukane  4096 Jun  5 14:54 library
drwxrwxr-x 10 enukane enukane  4096 Jun  5 14:54 plugins
drwxrwxr-x  3 enukane enukane  4096 Jun  5 14:54 src
"}
[1] {"time"=>1433484250, "command"=>"total 72
drwxrwxr-x  9 enukane enukane  4096 Jun  5 15:00 .
drwxrwxr-x 10 enukane enukane  4096 Jun  5 15:00 ..
-rw-rw-r--  1 enukane enukane     0 Jun  5 15:00 .empty
-rw-rw-r--  1 enukane enukane 15276 Jun  5 14:54 CMakeCache.txt
drwxrwxr-x  4 enukane enukane  4096 Jun  5 14:54 CMakeFiles
-rw-rw-r--  1 enukane enukane 11822 Jun  5 14:54 Makefile
drwxrwxr-x  2 enukane enukane  4096 Jun  5 14:54 bin
-rw-rw-r--  1 enukane enukane  2288 Jun  5 14:54 cmake_install.cmake
-rw-rw-r--  1 enukane enukane    46 Jun  5 14:54 exec.cfg
drwxrwxr-x  3 enukane enukane  4096 Jun  5 14:54 include
drwxrwxr-x  5 enukane enukane  4096 Jun  5 14:54 lib
drwxrwxr-x  2 enukane enukane  4096 Jun  5 14:54 library
drwxrwxr-x 10 enukane enukane  4096 Jun  5 14:54 plugins
drwxrwxr-x  3 enukane enukane  4096 Jun  5 14:54 src
"}
[2015/06/05 15:04:10] [ info] Flush buf 1658 bytes
(snip)