Issue 30/more args limit constant

Well, here's a crack at solving Issue #30:

```commit```
 	use LIMIT constant, support more options for Issue #30 …

-Switch to Getopt::Long to support full and abbreviated names
    -new options include:
        -s, --sort
        -t, --time
        -a, --after
        -h, --help

-set options in BEGIN to allow LIMIT constant to use $opt_l
    -DON'T limit the items per page as we don't know how many
        of those posts will be imgur links
```

Documentation was also added.  We can add more as we go.  @thedead would you mind testing this branch when you have time?