Add ICU encoding names

[mike42]

I tried to do some conversions via ICU, but it does not recognise the iconv code page names reliably.

For example, an 'ambiguous code page' warning is thrown for many entries, while some cannot be used at all (eg. CP774/CP1118 does not appear to be present)

Add a new icu entry to each entry in encodings.yml where ICU knows about the library.

References for correct names:

• http://demo.icu-project.org/icu-bin/convexp
• http://site.icu-project.org/charts/charset