added flag for retrieving parameters from the command line

[hjkatz]

• Updated the arguments to be passed as a hash for simplicity (all the defaults have been kept the same)
• Added new flag --parameters-cli which will prompt the user for parameters values at runtime

[hjkatz]

Please review the recent changes.

• changed flag name from --parameters-cli to --interactive (hopefully for a more encompassed interactive mode in the future)
• fixed a bug with custom parameters in the update step (similar to the create, but I forgot initially)
• decided not to do the additional "Use previous value" for parameters during the update

I looked into the amount of effort for attempting to use the old stack during an update step to get the differences in parameters interactively, but it proved too high (for now). It would require a restructuring with a more direct call to dsl compile or something similar. This is because the params are parsed immediately as they are called in the dsl, instead of parsing the dsl then asking for params. This ordering makes it very difficult to even add the interactive mode in the first place.

I would recommend a restructuring of the cfntemplate.rb file into 2 files. One for running, one for the template. Then you can add a compile/parse/extract step for all cli things first (such as params, tags, etc...), and after that you can use those parsed values to create the final template.