cli refactor (stdin, remove generate)

[chase-moskal]

this would allow cool people to

importly < echo "mobx;preact" > importmap.json

which should be possible

[chase-moskal]

consider removing --config option, favoring stdin as the only means by which to pass in a config

[chase-moskal]

also going to remove generate command, so now importly generate is just importly

so now that's:

• [x] accept stdin
• [x] remove --config
• [x] remove --host (should be in importly.config)
• [x] remove generate
• [x] reads importly.config and package.json