Add a filter by content option

[dadoonet]

You can filter out documents you would like to index by adding one or more
regular expression that match the extracted content.
Documents which are not matching will be simply ignored and not indexed.

If you define the following fs.filters property in your
~/.fscrawler/test/_settings.json file:

{
 "name" : "test",
 "fs": {
   "filters": [
     ".*foo.*",
     "^4\\d{3}([\\ \\-]?)\\d{4}\\1\\d{4}\\1\\d{4}$"
   ]
 }
}

With this example, only documents which contains the word foo and a VISA credit card number
with the form like 4012888888881881, 4012 8888 8888 1881 or 4012-8888-8888-1881
will be indexed.

Closes #463.