The after step fail screenshot is now named based on
feature file / step text + line number to make it easier to find

Added a new feature step called
 And show me a screenshot named "X"

which will save a screenshot to the dataroot
behat/html_output/screenshot/custom/ directory

This is useful to see snapshots at any point (not just fail), eg

 And show me a screenshot named "1"
 And I click on "XYZ"
 And show me a screenshot named "2"
 And I press "ABC"
 And show me a screenshot named "3"

Change-Id: I3446dd57a2c79d14c0e52239ce188a22d8ff09c7
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>