Enhancements for repetitive work

[rahul286]

Inside fm shell, we mostly do

• Set work dir to ~/frappe-bench.
• Set bench use <sitename> to the name of fm create <sitename> as all bench commands we use for that site. Might change once we have multisite.

On host machine

• fm reset <sitename> which does bench reinstall and restore sitename and setup config.

[Xieyt]

• Set work dir to ~/frappe-bench.

Implemented in #131.