support setting load options on instances

proof of concept

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)
    bs = relationship("B")

class B(Base):
    __tablename__ = 'b'

    id = Column(Integer, primary_key=True)
    a_id = Column(Integer, ForeignKey('a.id'))
    cs = relationship("C")

class C(Base):
    __tablename__ = 'c'

    id = Column(Integer, primary_key=True)
    b_id = Column(Integer, ForeignKey('b.id'))

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

sess = Session(e)
sess.add_all([
    A(bs=[
        B(cs=[C(), C()]),
        B(cs=[C(), C()])
    ])
])
sess.commit()

a1 = sess.query(A).first()
inspect(a1).load_options = inspect(a1).load_options.union([joinedload(B.cs)])

for b in a1.bs:
    print b.cs

im not sure why the above option doesn't have to mention "A.bs" at all, there should be some path logic taking this into account. The public API should be:

inspect(a1).set_load_option("bs", joinedload(...))

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