Add context manager methods to the FeatureDB class

[Fan-iX]

Hi,

I use a python web server to serve gff data with gffutils.

In my application, a FeatureDB instance is created on every web request, and I notice that I have to .close() the FeatureDB.conn explictly, or there could be a file descriptor leak / memory leak.

This PR add context manager methods __enter__ and __exit__ to the FeatureDB class, so that we can manage it within a with statement:

with sqlite3.FeatureDB("test.gff.db") as db:
    ...

instead of

try:
    db = sqlite3.FeatureDB("test.gff.db")
    ...
finally:
    db.conn.close()