PR:2 Pass in a list of command args as the second bash argument and xargs

[alanbacon]

(includes PR:1, can be merged instead of PR:1)

The passed in list will be checked for correctness, unpacked and appended to the orginal command

From updated readme:

---

Pass in arguments as an array::

>>> bash('ls', ['.'])
bash.pyc
tests.pyc

---

Because arguments can now be passed in as a list and because we can get the results of the previous call as a list: we can implement the xargs function. Where the results of the previous call are mapped onto the next function:

---

>>> bash('ls').bash('grep "\.py"').xargs('grep "author=\'Alex Couper\'"')
'setup.py:    author=\'Alex Couper\','

Which is similar to, but not exactly the same as (due to how grep works):

>>> results = [f for f in bash('ls').bash('grep "\.py"')]
>>> list(map(
              lambda f: [f, bash('grep "author=\'Alex Couper\'"' + f)]
         ))
[  [''setup.py', '    author=\'Alex Couper\',']  ]

---