make default --save semver option configurable

Implementing suggestion in npm/npm#4713

You can return old npm behaviour in config:
yapm config set save-range '~'
yapm install --save express

Or just use range shortcut in the command line:
yapm install --save --range='~' express

Available options for save-range are:
 - "~"
 - "^"
 - ">="
 - "<="
 - "=" - no range (exact)