Fix #17, #87: govet becomes SLOW linter by default

[jirfag]

1. Allow govet to work in 2 modes: fast and slow. Default is slow.
   In fast mode golangci-lint runs go install -i and go test -i
   for analyzed packages. But it's fast only when:

• go >= 1.10
• it's repeated run or $GOPATH/pkg or go env GOCACHE is cached
  between CI builds
  In slow mode we load program from source code like for another linters
  and do it only once for all linters.

3. Patch govet code to warn about any troubles with the type
   information. Default behaviour of govet was to hide such warnings.
   Fail analysis if there are any troubles with type loading: it will
   prevent false-positives and false-negatives from govet.

4. Describe almost all options in .golangci.example.yml and
   include it into README. Describe when to use slow or fast mode of govet.

5. Speed up govet: reuse AST parsing: it's already parsed once by
   golangci-lint.
   For "slow" runs (when we run at least one slow linter) speedup by
   not loading type information second time.

6. Improve logging, debug logging

[jirfag]

@alecthomas, users of golangci-lint: what do you think about this fix?

Are you ok that govet will become a slow linter: it won't be enabled with --fast and will run much slower if it's enabled explicitly with only "fast" linters (golint, gofmt, gocyclo)? What do you think about "fast" mode by enabling option use-installed-packages: true?

[alecthomas]

This works for me 👍

[jirfag]

thank you