Update synhighlithing for all buffers

[LucHermitte]

When tags_options.auto_highlight is true, the syntax highlighting is updated only in the current buffer. Other buffers belonging to the same project aren't updated.

Need to:

• Either update on BufEnter is the time stamps of the tags updating is > last time the buffer has been accessed, or update every buffer displayed when the tags are updated (this can be very slow)
• Load the syn highlighting in the other files of the project when loaded for the first time.