feat: add parse options to ignore separators from signature

[obedparla]

Solves this issue: #41

I encountered this issue when parsing some property listing website emails, where they use --- as separators within their emails, e.g a Zoopla.co.uk email will look like this (shortened version).

The parse method now had options to alter its behavior, and I added solely excludeSignatureSeparators.

Dear team at XYZ
You have a new=20
    Tenant enquiry.

TENANT ENQUIRY FROM XYZ VIA ZOOPLA

Please find below an enquiry from XYZ
Please respond to this enquiry promptly to maximise your chances of converting this lead.

---

APPLICANT'S DETAILS

......

---

Full address 

.....