fix slow vol set. move user setting to cfg file. message prints optional

[vegadj]

Hi there,

I have added some small features and minor changes.

• VOLUME_INCREMENT variable moved into config file for user configuration.
• VERBOSE_MODE variable added in config file. This bool variable adjust non informative commands to print nothing into screen. like vol up and vol down next etc.
• all information like echos changed into new vecho function to take have control with VERBOSE_MODE variable. Function variables is set as local variables.
• vol commands adjusted for speed. every osascript calls takes about 0.25 sec. When calling vol set, vol status is also called which has no effect on vol set. (it just need for vol up and vol down and vol). This makes vol set calls slow which takes 0.750 sec total.

Some minnor suggestion that I thought but did not change in this commit.

• cfg file could create by separate install script. There is no need to check and create cfg file every time the script executed.
• there is no need to check spotify is running or not. Eventually it starts itself. This also slows down the script execution.