add support to create custom filters

custom filters can be created by providing a filters hash. For each
filter a config file will be created in the filters.d directory.
filterregex and ignoreregex can be provided as string or array.

example

default['fail2ban']['filters'] = {
  'nginx-noscript' => { 'filterregex' = ['^<HOST> -.*GET.*(\.php|\.asp|\.exe|\.pl|\.cgi|\scgi)'] }
}
update README with notes about custom filters
Merge pull request #16 from bumi/master

Support to create custom fail2ban filters