[option] fix print helper msg twice bug, close #37

[benja-wu]

• In pflag pkg, line 1129 says The return value will be ErrHelp if -help was set but not defined. So in the original implementation, we use this err type as a signal for us to print the usage message.
• Before pflag.Parse() return ErrHelp, it actually prints to the stdout with function usage line 927
• So this is the reason why it will double-print the usage message.

Here is what the solution has done:

1. Add helper define. So it won't return ErrHelp.
2. In our option.Parse() function, the function's action change to if pflag parsing found error, then return this err and print to std, exit at last.

Note the original implementation treats pflag's other type return error as a normal condition, and allows Easegress to continue processing. I would like to know the former design's consideration.