Plot improvements

I added a couple of improvements, mainly related to plotting:


- command-line of profiled command is now stored into profile file. It is displayed by default as graph title
- memory usage recorded when entering and leaving functions are now integrated on curves, instead of just being shown with brackets.
- added an option (-t) to `mprof plot` to manually set the figure title
- added an option (-n) to `mprof plot` to prevent display of function timestamps
- empty profile files are now properly handled by plot, a clear error message is displayed to the user.

Plus some code cleanup.