Update regular expression for checksums to match package structure

[tow8ie]

Without this fix of the regex, the validate-checksums function will only update the resources in the boot-cljsjs-checksums.edn file which are matched by the incomplete regex. This leads to deletion of entries in boot-cljsjs-checksums.edn.

The regex that describes which files have checksums now reflects:

• The cljsjs package structure uses development as directory name for dev resources (documented here).
• Resources that are checked do not only have an .inc.js file ending, the .ext.js resources are checked, too (see here or here, for example).

Also, the regex that is displayed in the help message of validate-checksums is now indentical to the one actually used.

[Deraen]

I have simplified the regex to now care about the directory, but only include .inc.js files, as extern files are already managed in Git, their checksum doesn't matter that much. https://github.com/cljsjs/boot-cljsjs/commit/3c2bb4ef46c3fe7252fc398b1c3b5572136e7b42