Feature/exact semver

Modified version of @isaacs [suggestion](https://github.com/npm/npm/issues/4713#issuecomment-35423346):

>  I would be ok with adding a single --save-exact | -E boolean option, which causes it to always save the exact version number installed. Patch welcome for that.

Implemented as a single `--exact` option rather than `--save-exact` to be combined with the `--save`, `--save-dev` and `--save-optional`. Otherwise, for completeness it this would need introduce to be 3 new options: `--save-exact`, `--save-dev-exact` and `--save-optional-exact` which doesn't seem ideal.