shellutils: args parameter for run_shell_command()

• New args parameter for run_shell_command() so that clients can pass shell command arguments that will be escaped and substituted. Usage is quite like run_sql(query, params).

• Updated run_shell_command() clients, since args is in the second position.

• Added test cases for run_shell_command().

• Improved documentation of shellutils to fit the epytext markup.