Add g:easytags_suppress_report option

Currently, easytags reports the tag file update status. This feature is disabled by default on async mode, but is enabled on sync mode. The problem is that this message overwrites other vim messages. As a result, if I save a file and switch into insert mode, I cannot see `--INSERT--` string.

`g:easytags_suppress_report` works pretty much similar to `g:easytags_suppress_ctags_warning` option. If this variable is set and true, easytags does not print the update status.