Add more command line options for install.sh

New command line options:
 * -b, --backup     create/overwrite backup files
 * -v, --verbose    verbose mode
Modify HISTIGNORE env var

Remove "* --help" as it might actually be handy to have in history.
For example, if the full help text consists of dozens of lines
and the user wants to grep lines with a certain word in them,
the user doesn't have to write the "command --help" part again.