Support for user-defined long argument prefix

[germangh]

Some programs use prefixes other than -- to identify long arguments. Example:
packer. This adds support for parameter _long_prefix that can be used to define
the prefix for long arguments.

[amoffat]

looks good 👍 . this will go in the 1.12 release. don't worry about the failed test..doesn't look releated to you.

[Ellon]

Looks like the _long_prefix option does not work with keyword arguments without value.

For example, this calls the expected (and incorrect on purpose) command:

In [27]: sh.cat(myarg=2,_long_prefix='-')
---------------------------------------------------------------------------
ErrorReturnCode_1                         Traceback (most recent call last)
<ipython-input-27-81d4f4c42ea2> in <module>()
----> 1 sh.cat(myarg=2,_long_prefix='-')

/home/emendes/.local/lib/python3.4/site-packages/sh.py in __call__(self, *args, **kwargs)
   1425             stderr = open(str(stderr), "wb")
   1426 
-> 1427         return RunningCommand(cmd, call_args, stdin, stdout, stderr)
   1428 
   1429 

/home/emendes/.local/lib/python3.4/site-packages/sh.py in __init__(self, cmd, call_args, stdin, stdout, stderr)
    772 
    773             if should_wait:
--> 774                 self.wait()
    775 
    776 

/home/emendes/.local/lib/python3.4/site-packages/sh.py in wait(self)
    790 
    791             else:
--> 792                 self.handle_command_exit_code(exit_code)
    793 
    794                 # if an iterable command is using an instance of OProc for its stdin,

/home/emendes/.local/lib/python3.4/site-packages/sh.py in handle_command_exit_code(self, code)
    813             exc = exc_class(self.ran, self.process.stdout, self.process.stderr,
    814                     ca["truncate_exc"])
--> 815             raise exc
    816 
    817 

ErrorReturnCode_1: 

  RAN: /bin/cat -myarg=2

  STDOUT:


  STDERR:
/bin/cat: invalid option -- 'm'
Try '/bin/cat --help' for more information.

But this does not:

In [28]: sh.cat(myarg=True,_long_prefix='-')
---------------------------------------------------------------------------
ErrorReturnCode_1                         Traceback (most recent call last)
<ipython-input-28-89700f6cc61e> in <module>()
----> 1 sh.cat(myarg=True,_long_prefix='-')

/home/emendes/.local/lib/python3.4/site-packages/sh.py in __call__(self, *args, **kwargs)
   1425             stderr = open(str(stderr), "wb")
   1426 
-> 1427         return RunningCommand(cmd, call_args, stdin, stdout, stderr)
   1428 
   1429 

/home/emendes/.local/lib/python3.4/site-packages/sh.py in __init__(self, cmd, call_args, stdin, stdout, stderr)
    772 
    773             if should_wait:
--> 774                 self.wait()
    775 
    776 

/home/emendes/.local/lib/python3.4/site-packages/sh.py in wait(self)
    790 
    791             else:
--> 792                 self.handle_command_exit_code(exit_code)
    793 
    794                 # if an iterable command is using an instance of OProc for its stdin,

/home/emendes/.local/lib/python3.4/site-packages/sh.py in handle_command_exit_code(self, code)
    813             exc = exc_class(self.ran, self.process.stdout, self.process.stderr,
    814                     ca["truncate_exc"])
--> 815             raise exc
    816 
    817 

ErrorReturnCode_1: 

  RAN: /bin/cat --myarg

  STDOUT:


  STDERR:
/bin/cat: unrecognized option '--myarg'
Try '/bin/cat --help' for more information.

Is this the expected behavior?