New: Basic CLI with subcommands

[avcopan]

Currently the only meaningful command is expand, which runs the ste_mech.py script to expand stereochemistry:

mechanalyzer expand

An example is included under examples/expand

To see the available subcommands, do:

mechanalyzer --help

To test that things work, you can use the greetme command to print "Hello, world!" to the screen:

mechanalyzer greetme