[commands] Add more i18n properties for HelpFormatter
[commands] Add more i18n properties for HelpFormatter

removed fixed strings "Commands:" and help page ending note.
and added properties modify these strings.
default behavior is not changed. fix #1886

Harmon758/discord.py,imayhaveborkedit/discord.py,rapptz/discord.py,Harmon758/discord.py,Rapptz/discord.py,khazhyk/discord.py