Make require('iconv-lite') fast

Requiring iconv-lite is costly due to the library loading the encodings. This fixes that by lazy-loading them.

Before:
````
node -e "require('node-iconv')"  0.18s user 0.02s system 98% cpu 0.204 total
```

After:
```
node -e "require('node-iconv')"  0.07s user 0.01s system 97% cpu 0.081 total
```