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.

I believe you would need to change the module line 1 in go.mod back to module github.com/hennedo/escpos; I can either do that for you on this PR if everything else looks OK and then you can pull the single commit, or you can accept the PR and make subsequent commit to revert line 1 of your go.mod.

Fixes #11

[justinmichaelvieira]

Closed in favor of #14 (I had accidentally put my fork as the module in go.mod, PR14 fixes that).