Use golang native iconv library instead of gnu iconv

[justinmichaelvieira]

@hennedo, when you get a chance, can you take a look at this?

Removes github.com/qiniu/iconv in favor of github.com/justinmichaelvieira/iconv.

https://github.com/justinmichaelvieira/iconv is my fork of (current) https://github.com/mushroomsir/iconv, with CP850 support added (I would wager CP850 support was what you saw missing, when you attempted the same, so I added it in and added a test case on my fork; Opened a PR there as well, but for now the fork seems to work).

mushroomsir/iconv@6b19bf0 is the relevant commit adding CP850 to go native iconv lib.

Fixes #11

[justinmichaelvieira]

@hennedo just pinging you as a gentle reminder to please take a look if you get a chance. I think removing the CGO dependency greatly increases the utility of this lib.

[justinmichaelvieira]

@hennedo this should be very safe to merge at this point, a few other developers in this repo have said they have tested and are using my fork, which is equivalent to this PR, currently, just so they can save themselves hassle of compiling the gnu iconv dependency, and I've been using it successfully for >1yr at this point without issue.