Allow :join_table association option to have a string value
Allow :join_table association option to have a string value

Before, using a string value would result in very weird error
messages, because of the implementation of the many_to_many
add_/remove_/remove_all_ methods.  Have those methods use
database.dataset.from(join_table) instead of
database[join_table], which should work.  The documentation will
remain the same, and I don't guarantee that using a string will work
in future versions.  The main reason for this commit is so if you do
use a string, you won't get an error message that doesn't make sense.

uhoh-itsmaciek/sequel,offshore-safety/sequel,davidbegin/sequel,tmm1/sequel,uhoh-itsmaciek/sequel,Marketcircle/sequel,offshore-safety/sequel,mpalmer/sequel,davidbegin/sequel,t-morgan/sequel,reubano/sequel,yazshel/sequel,dennis-d/custom_sequel,knut2/sequel,pfarrell/sequel,nju520/sequel,gferguson-gd/sequel,dennis-d/custom_sequel,gferguson-gd/sequel,beni55/sequel,mpalmer/sequel,chef/sequel,offshore-safety/sequel,freshpay/sequel,knut2/sequel,seeARMS/sequel,beni55/sequel,t-morgan/sequel,nicohvi/sequel,bigfix/sequel,pfarrell/sequel,nicohvi/sequel,seeARMS/sequel,yazshel/sequel,freshpay/sequel,nju520/sequel,reubano/sequel