Add support for cargo's --config arg

[TimJentzsch]

Objective

Closes #201.

Allows users to quickly try out different configurations without modifying Cargo.toml.

For us, it's most useful for finding the best default config for the web profiles.

Solution

Add support for --config argument for bevy build and bevy run, analogue to the cargo counterpart.
This essentially allows you to modify the Cargo.toml without editing the file, e.g. --config "profile.web.debug=false".

We already exploit cargo's --config arg to configure our default web compilation profiles.
So we have to ensure that the user-provided args overwrite our default ones.

To do this, we change the default web profiles to be prepended to the user-provided --config args, instead of converting them to --config arguments directly.
Since --config is resolved left-to-right, the defaults will be overwritten by the user.

Testing

Try for example:

bevy build --config "profile.web.debug=false" --config 'profile.web.inherits= "release"' web

Now instead of

Finished `web` profile [unoptimized + debuginfo] target(s)

you should get

Finished `web` profile [optimized] target(s)

Debug info removed and optimizations enabled!
(Of course this can be done easier via bevy build --release web, but like this it's easier to benchmark different compilation profiles to choose the best default)

[DaAlbrecht]

Works like a charm! Tested with config options as arguments or by passing a file.
If the comment about the cargo build options is resolved this can be merged IMO 😊

[DaAlbrecht]

Perhaps a test case for add_values_separately would be nice

[DaAlbrecht]

Nice work!