Added replace plugin

I've added a plugin that allows you to replace strings or regex matches in a HTML document. At the moment it basically has two methods for replacing:

1. Simple string replace. Provide the string you want to search for (--search-str) and replace it with the value you provided (--replace-str)
2. Regex sub. You can provide the plugin with a file (--regex-file) that contains two parts per line - separated by a tab. The first part is a regex. The second part is the value that is used to replace the match. You can grab matching groups from the regex and use them in the value that is used to replace the match. An example of a line is `http://.+/(.+\.iso)	http://evilsite.com/\1`.

I've also added a list with available plugins in the README. Not sure if you find this useful. I personally always like a list that tells me what I can do with a tool.