use an ignorefile to prevent unwanted email addresses being added

The ignorefile is a file of regular expressions (including plain
strings).  For every email address being added, it is checked against
all the regular expressions.  If it matches any then the email address
is skipped.

The ignorefile is set by the ignorefile value in the notmuch config file
in the [addressbook] section, if present.
ignorefile can now have static strings and/or regex

regex should start and end with /

So an ignore file could have stuff like:

noreply@
autoshipping@amazon
/@.*\.subdomains\.com/
/-test.*@/
Allow blank lines and comments in the ignore file

comments should have # as the first non-whitespace character
Add delete action
clean up code

- move ignorer into the db class
- break up the import class
Make actions more consistent in naming and arg list
reduce spreading code over multiple lines
remove stray quotation mark
fix UnicodeEncodeError when printing matches