cmd/test-coverage: implement a new command assessing code coverage on a whole repo

[sbinet]

right now, this is done with a shell script:

• https://github.com/gonum/tools/blob/081df40a91e7b56c006fed3597ed4e952c64427a/.travis/test-coverage.sh

that shell script is working around the fact that go test -coverprofile=profile.out can only work for a single package (and not for a bunch of packages, like go test ./... does)

rewriting that script in Go could be helpful (and be reused by other projects.)

[ahlusar1989]

@sbinet I'll take a look at this

[ahlusar1989]

@sbinet Is this something that could be used to traverse each path and amalgamate the profiles:

https://github.com/go-playground/overalls

[sbinet]

Hum... Yes, probably :)
(Then this issue is probably just about modifying our scripts to use that.)

[ahlusar1989]

Sounds good @sbinet - on it!