add INSERT IGNORE syntax for mysql

[Mike Bayer]

if we stick with what ants has been doing for postgres, it would look like:

table.insert(mysql_ignore=True)

[jek]

Was added to 0.4.4 as table.insert(prefixes=['IGNORE']('IGNORE')) or table.insert().prefix_with('IGNORE')

[Mike Bayer]

Removing milestone: 0.4.4 (automated comment)