stbt.press: new configurable parameters for function call

This commits adds 3 new parameters that we can specify to press
function:
 - verbose (default: True), inform function that after sending a
   key press, it should be display on screen as well
 - verbosity_duration (default: 3.0), previously hardcoded and constant,
   now can be set globally in config file (function draw_text is still
   using original variable name `duration_secs`, name change affects
   only function `press`, press never exposed this variable as it's
   argument to external scope)
 - show_timestamp (default: False), function while being verbose will
   also display timestamp when given command was received

To achieve that I had to improve config_parser to recognize boolean
values, previously we could only use strings, floats and integers.
stbt.Display: New config parameter added to Display class

If user specify in configuration file for section [global], variable
called show_clock (default: False), recorded video will have clock in
right top corner. Feature useful when analyzing video against logs
and/or texts with commands that are displayed in left top corner.