[ADD] possibility to use %FILE% and %LINE% in commands

This pull request allows to use the filename and the line number in the command line.

Use case: 
tig blame Terminal.py +53
open the blame view of file Terminal.py at line 53

This can be configured in the commands with:
{ "keys": ["ctrl+shift+b"], "command": "open_terminal", "args": {"parameters": ["-e", "tig blame %FILE% +%LINE%", "--working-directory=%CWD%"]} },