support setting load options on instances

Migrated issue, originally created by Michael Bayer (@zzzeek)

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(...))

Start with the shown inspect(a1).load_options proof of concept and the proposed inspect(a1).set_load_option API; trace how the instance and relationship path are represented. Done means a supported instance-level load option can target the intended relationship path, with behavior matching the example.