adds more checks for alt text, removes whitespace, fixes false positive on alt=""

[ecerta]

Fixed the following Issues:

https://github.com/mvdschee/web-accessibility/issues/6

Added the following Features:

• No longer flags alt="" as false, since this is a best practice for purely decorative images, and alters error message accordingly
• Checks for bad alts like "image" and "picture"
• Checks for bad alt beginnings, like "image of" and "picture of"
• Adds a length check for alts, since most screen readers cut off after 125 characters
• Removes some extraneous whitespace found in the course of working on this issue

[ecerta]

Also, I saw the PR guidelines said to add tests for all new code but I couldn't find a test suite. Is it hidden in an unusual place, or is a test suite on the list for future improvements?

[mvdschee]

The test suite is definitely on the to do list.