Allow to pass :extra_args to mysqlstream

[thbar]

See #89 for the background - it's fairly common to have a default client encoding not set to UTF-8 and as such, the data will choke as not UTF-8 when using mysqlstream.

What we could do to smooth this:

• add an :extra_args option to mysqlstream
• if passed, this should be added to the mysql command
• allow to write this:

# we should still support this
source :in, :type => :database, :mysqlstream => true
# but we should also allow that
source :in, :type => :database, :mysqlstream => { :extra_args => "--default-character-set=utf8" }