add option to YML config for "dev_mode"

[justin808]

How about an option for:

dev_mode: dev-server | static | DLL

dev-server uses the normal live reload and is the default, and does not require the manifest.
static uses the normal fingerprinted, manifested assets.
*DLL uses the webpack DLL which does not fingerprint the assets and does not uses a manifest.

See shakacode/webpacker_lite#20.

[gauravtiwari]

@justin808 How this would help?

To me, this will complicate things - three modes just for development environment?
With webpacker the first two modes are pretty trivial to use - just switch the binstubs however I see the DLL mode requires some setup and bit complicated to use.

Probably leave this to project owners?

[justin808]

@gauravtiwari, It turns out that when using the DLL, some assets need to be hashed. So we can close this ticket.