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
remove help sub, add documentation to script
Merge pull request #31 from aggrolite/issue-30/more-args-limit-constant

Issue 30/more args limit constant