Add "18" to AN_REQUIRING_PATTERNS

[fatcatt316]

For example, you'd want "An 18 point lead" instead of "A 18 point lead".

[rossmeissl]

@fatcatt316 thanks for your PR! A couple of comments:

• Can you please include a testcase?
• I'm not sure this is as complete as it can be. For example, consider the phrase "a 183-toed marmot" which this patch would incorrectly present as "an 183-toed marmot."

Thanks again!

[jongaegler]

A_REQUIRING_PATTERNS = /^(([bcdgjkpqtuvwyz]|onc?e|one(armed|time)|ouija|oaxaca(n|nia|nthaxia)?)$|e[uw]|uk|ub[io]|ufo|ur[aeiou]|use|ut([^t])|unani|uni(l[^l]|[a-ko-z]))/i
AN_REQUIRING_PATTERNS = /^([aefhilmnorsx]$|hono|honest|hour|heir|herb|[aeiou]|8|1[18](\d{3})*(\D|$))/i

I'm having trouble pushing to this repo ¯_(ツ)_/¯ but here are the regex catching those cases, as well as optimizing some of the searches in javascript. I think it translates exactly in ruby