Allow manual parameter and variable definition

[rjmholt]

The most magical part of PSArm at the moment is the way ARM parameters and variables are created. We should also allow users to create these objects easily (perhaps with New-PSArmVariable and New-PSArmParameter -- these could even create the variables as PowerShell variables themselves). There are some problems to solve here around not needing to respecify the variable name and knowing when we create the template which variables were used, but I think they should be solvable.