ICU-21248 Adds source file check (UTF-8 and absence of BOM) to

ICU-21248 Adds source file check (UTF-8 and absence of BOM) to
Travis Continued Integration.

ICU-21248 Factors in review comments.

ICU-21248 Changes: no more filtering of markdown files, moved the
script to icu/tools/script/ directory, removed BOM from one
README.md file.

ICU-21248 Adjusts path to the icu-file-utf8-check script.

ICU-21248 Extends coverage of UTF-8/BOM check to all of icu/ directory.