Address #74 - add new-exe-project subcommand

[sogaiu]

This PR is an attempt to address #74 by adding a new-exe-project subcommand.

It includes changes to jpm/scaffold.janet and jpm/commands.janet.

jpm/scaffold.janet:

• Addition of a new template named exe-project-template
• Change the logic a bit in scaffold-project to accomodate the new template

jpm/commands.janet:

• Modify the usage string in help to mention the new subcommand
• Addition of a new-exe-project function that invokes scaffold/scaffold-project appropriately
• Add key-value pair for new subcommand in subcommands definition

[bakpakin]

LGTM.