Allow using --watch option in patternlab build

[drhossa]

Summary of changes:

• core/src/lib/patternlab.js: reset patterns before updating
• core/src/lib/watchPatternLabFiles.js: add unlink events to patternWatcher
• cli/bin/build.js: allow passing the watch flag

[bmuenzenmeyer]

this looks awesome! hope to review soon

[stale]

It's hard to keep track of everything. This issue has been automatically marked as stale because it has not had recent activity, neither from the team nor the community. It will be closed if no further activity occurs. Please consider adding additional info, volunteering to contribute a fix for this issue, or making a further case that this is important to you, the team, and the project as a whole. Thanks!

[stale]

It's hard to keep track of everything. This issue has been automatically marked as stale because it has not had recent activity, neither from the team nor the community. It will be closed if no further activity occurs. Please consider adding additional info, volunteering to contribute a fix for this issue, or making a further case that this is important to you, the team, and the project as a whole. Thanks!

[mfranzke]

Fight the bot

[JosefBredereck]

Need to mention
Closes #887