Refactor mso xml detection for trailing signatures

Zip archives do not care about the order of the files inside them. This
makes the signature files of Ms office formats to be in the trailing
part of the archive and not get picked up by the detection alg. This
commit improves detection by giving up search for general MS office
signature search in favor of searching for more precise entries.
Related issue: gabriel-vasile#81

Add docs for EqualsAny

Improve zip sub formats detection

Add Performance section in README

Use Is method in tests instead of string comparƣson

Behave as http.DetectContentType and return text/plain for empty input

Merge pull request gabriel-vasile#83 from vansante/add-guard-for-msof…

…fice-slice

Add a guard for msoffice byteslice index

Add support for UTF-16

Add more v1.x changes in readme

Merge branch 'fix_msoxml'

Fix heic and heif extensions