Flag to control subject encoding

[begoon]

The library always encodes the email subject to quoted-printable to conform to the standards.

Unfortunately, some popular UI clients, for example, Gmail, do not display quoted-printable subjects correctly. Instead, though, they deal perfectly with non-ascii values in the subject, displaying them correctly.

This PR adds an option to disable quoted-printable encoding for the subject. The default behaviour, when the option is not provided, remains the same.