feat(cli): Improve vector generate syntax and formatting

[Jeffail]

This PR changes the vector generate syntax to allow forward slashes: foo/bar,baz/boo, closing #1684. This is backwards compatible as the pipe syntax can still be used.

The output now also adds empty lines between component types to make the formatting nicer:

data_dir = "/var/lib/vector/"
dns_servers = []

[sources.source0]
max_length = 102400
type = "stdin"

[transforms.transform0]
inputs = ["source0"]
drop_field = true
drop_invalid = false
type = "json_parser"

[sinks.sink0]
healthcheck = true
inputs = ["transform0"]
type = "console"

[sinks.sink0.buffer]
type = "memory"
max_events = 500
when_full = "block"

This is the lowest hanging fruit of #1685, we're still missing:

1. Proper ordering of fields (inputs first, followed by type, and so on)
2. Nicer indentation
3. All required and common fields for all components

But this stuff would require a fair amount of time investment, and I'd like to have a plan/spec for #1735 before we go ahead with it. I'll update #1685 accordingly.

[Hoverbear]

@Jeffail Are docs updates coming in another PR?

[Hoverbear]

Looks so much better!

$ cargo run -- generate stdin,kafka/add_fields,remove_fields,json_parser,json_parser/console
    Finished dev [unoptimized + debuginfo] target(s) in 0.14s
     Running `target/debug/vector generate stdin,kafka/add_fields,remove_fields,json_parser,json_parser/console`
data_dir = "/var/lib/vector/"
dns_servers = []

[sources.source0]
max_length = 102400
type = "stdin"

[sources.source1]
type = "kafka"

[transforms.transform0]
inputs = ["source0", "source1"]
type = "add_fields"

[transforms.transform1]
inputs = ["transform0"]
type = "remove_fields"

[transforms.transform2]
inputs = ["transform1"]
drop_field = true
drop_invalid = false
type = "json_parser"

[transforms.transform3]
inputs = ["transform2"]
drop_field = true
drop_invalid = false
type = "json_parser"

[sinks.sink0]
healthcheck = true
inputs = ["transform3"]
type = "console"

[sinks.sink0.buffer]
type = "memory"
max_events = 500
when_full = "block"

[Jeffail]

@Jeffail Are docs updates coming in another PR?

Not for this one, not sure there's any mention of generate on the site yet (won't be the case soon, after my configs guide)

[Hoverbear]

Ok! :)