Move styles to separate stylesheet, included via Javascript

[nfreear]

Hi @BernhardBehrendt,

As I said previously (#4), I think it would be beneficial to move the CSS styles from style HTML attributes, to a separate stylesheet.

The stylesheet would be injected/ included via Javascript.

Benefits:

1. Easier to override styles;
2. Possible to style on-focus and on-hover;
3. (Possible to lint/ validate the CSS).

This involves changes to the API ... the following configuration options would no longer work:

• buttonBgColor
• buttonTextColor
• noticeBgColor
• noticeTextColor
• linkColor

So, it will probably require a major version increment (semver).

I'll do a pull request shortly, and you can see what you think!

Yours,

Nick