Add an option to excluse some reserved words

[svengau]

Hi,

it would be nice to have an option to exclude some words from the slug generation. For example, with contact, blog, help:

testSchema.plugin(URLSlugs('username', exclude: ["contact", "blog", "help"]));

If you like it, I will try to propose a pull request for that.

[talha-asad]

Yes, sure looks like a good idea.

[svengau]

Here is the PR: #40
thanks