Overridable help text generation

[ty-i3]

Howdy! The newly-added section in the README.md has the details. This change brings cli-matic closer to par with urfave/cli and others..

Notes:

• The roles of :app :description and :app :global-opts might have changed slightly.
• I felt that cli-matic/core.clj could be decomposed into smaller pieces, with at least a new off-shoot for help text generation, perhaps cli-matic/help.clj.
• No spec's are added; not sure how you want to approach that.
• Didn't add any tests; run-cmd directly printlns the generated help text.

[l3nz]

Can you open up a bug so that there is a rationale and an example of usage for the change?

In general I would like to use a separate ns for help generation, core is getting too big.

[ty-i3]

Ok! Referencing #45

[l3nz]

Will be in 0.1.18