Allow excluding file patterns when generating component definitions

[pbugnion]

Thanks a lot for making Dash -- we have found it very useful for building dashboards.

We ran into an issue migrating to the new dash-generate-components utility.

In dash-bootstrap-components, we keep the components in src/components. Besides components files, this also includes a few unit test files called, e.g. src/components/__tests__/DropdownMenu.test.js. When we run dash-generate-components ./src/components <output>, these test files also get picked up. This leads to a traceback in the build logs:

Error with path src/components/__tests__/DropdownMenu.test.jsError: No suitable component definition found.
Error: No suitable component definition found.
at parse (/project/pascal/dash-bootstrap-components/node_modules/react-docgen/dist/parse.js:84:9)
at Object.defaultParse [as parse] (/project/pascal/dash-bootstrap-components/node_modules/react-docgen/dist/main.js:66:30)
at parseFile (/opt/anaconda/envs/Python3/lib/python3.6/site-packages/dash/extract-meta.js:64:51)
at dirs.forEach.filename (/opt/anaconda/envs/Python3/lib/python3.6/site-packages/dash/extract-meta.js:84:17)
at Array.forEach (<anonymous>)
at collectMetadataRecursively (/opt/anaconda/envs/Python3/lib/python3.6/site-packages/dash/extract-meta.js:79:14)
at dirs.forEach.filename (/opt/anaconda/envs/Python3/lib/python3.6/site-packages/dash/extract-meta.js:82:17)
at Array.forEach (<anonymous>)
at collectMetadataRecursively (/opt/anaconda/envs/Python3/lib/python3.6/site-packages/dash/extract-meta.js:79:14)
at componentPaths.forEach.componentPath (/opt/anaconda/envs/Python3/lib/python3.6/site-packages/dash/extract-meta.js:15:5)

While the error is, of course, legitimate, it would be nice to be able to suppress these tracebacks. I can see three routes:

• do nothing -- after all, this doesn't stop us from building metadata.json, it just makes it slightly more confusing.
• in dash/extract-meta.js, we could explicitly blacklist __tests__ directories in the same way that files that don't end in jsx? are black-listed. AFAICT, the __tests__ directory structure is the default with jest. react-docgen ignores node_modules, __tests__ and __mocks__ by default, so there is definitely a precedent.
• add a --ignore argument to dash-generate-components that allows passing file globs to be excluded.

Very happy to submit a PR if you decide on what the best course of action is.