Add build watch mode

[nandorojo]

Feature request

I'd love to be able to run bob build --watch, and for it to rebuild every time there is a change.

Why it is needed

I'm using bob in a monorepo, and I have multiple example apps (next, expo, gatsby) that need to update when I make changes. Currently I need to run bob build each time.

Possible implementation

I'm not sure how other repos do this, but whenever there is a file saved, it could run bob build.

Code sample

- bob build
+ bob build --watch

[nandorojo]

Original issue where this is concerned is from my library Dripsy: nandorojo/dripsy#32

[krizzu]

Hey,

This is a duplicate of #81 - have a look there for Satya suggestion on that.