Provide a command line option to specify analysis of a single function body

[hermanventer]

Issue

When debugging MIRAI in a context where a small repro is not readily available, it is helpful to be able to run MIRAI in a mode where it will only analyze a specified method.

Typically, the summary cache from a full run will still be available, so analysis of a function body in this way can be done with much more context than is convenient to provide in a small test case.

To implement this feature, add a command line option (see also #112) and then add a check to the after_analysis function in checker/src/callbacks.rs that filters out any function that is not the one specified by the command line option.