add composer user installer

[viceice]

https://getcomposer.org/doc/faqs/how-to-install-composer-programmatically.md

ref: renovatebot/renovate#2355

Idea:

• add ~/bin to path
• download composer-setup to ~/bin/composer-setup (make executable)
• renovate runs composer-setup --install-dir=$HOME/bin --filename=composer [--version=1.10.0] as preCommands

@rarkins should we add a wrapper script for composer-setup, so renovate only has to run install-composer 1.10.0 ?

[rarkins]

Good idea.