Support regular expression contains instead of 'a in b'

[memborsky]

Use re.search() to check if the value being compared is contained within the filter field.

>>> from jsonpath_ng.ext import parse
>>> 
>>> simple_dict = {
...   "phone_numbers": [
...     {
...       "type"  : "iPhone",
...       "number": "0123-4567-8888"
...     },
...     {
...       "type"  : "home",
...       "number": "0123-4567-8910"
...     },
...     {
...       "type"  : "home",
...       "number": "1098-7654-3210"
...     }
...   ]
... }
>>> 
>>> expression = parse('$.phone_numbers[?(@.type =~ "^hom")]')
>>> 
>>> [x.value for x in expression.find(simple_dict)]
[{'type': 'home', 'number': '0123-4567-8910'}, {'type': 'home', 'number': '1098-7654-3210'}]
>>> 
>>> expression = parse('$.phone_numbers[?(@.number =~ "[0-9]{4}-4567-[0-9]{4}")]')
>>> 
>>> [x.value for x in expression.find(simple_dict)]
[{'type': 'iPhone', 'number': '0123-4567-8888'}, {'type': 'home', 'number': '0123-4567-8910'}]

[coveralls]

Coverage remained the same at 0.0% when pulling 7ff235a on memborsky:add-regular-expression-contains-support into d4b8b87 on h2non:master.