Add the ability to define global selectors

[gsaray]

For example, to use footer or header in one place and use it globally

[jgermyn]

From Steve:

We could have the global selectors be defined in another file somewhere.

i.e.

header, footer, etc

The elementsPresent assertion could merge the global selectors with the selectors defined in the command itself. We could have an extra parameter on the end of the test that determines whether we want to skip merging the selectors.