Added custom user agent support on rss.Parser

[GaruGaru]

Related to #74
Some feed such as reddit's rss feed provide a very low request rate for bots, it would be nice to have an easy to use way to set a user defined user agent for the parser

[coveralls]

Coverage decreased (-0.001%) to 60.583% when pulling add64f6 on GaruGaru:master into eb870fd on mmcdole:master.

[coveralls]

Coverage decreased (-0.001%) to 60.583% when pulling add64f6 on GaruGaru:master into eb870fd on mmcdole:master.

[coveralls]

Coverage decreased (-0.001%) to 60.583% when pulling add64f6 on GaruGaru:master into eb870fd on mmcdole:master.

[bogatuadrian]

@mmcdole Will this be merged? Is it because coverage cc @GaruGaru?

[OrKoN]

I think it does not add much value: one can always use http to fetch and parse with fp.Parse. It's even a good idea to implement your own download function to handle network errors/retries/headers/etc better

[lmas]

Would still love to have this option for when making quick prototypes and stuff.