Allow current working directory to be the data directory.

[urbanslug]

We want to to allow for the configDir to be allowed to act as the data directory when the user choses this is through adding a field dataDirAsConfigDir :: Bool to SessionConfig.

[snoyberg]

@mgsloan since you're already working on this codebase, would you review this PR? @urbanslug is working on the updated yesod devel which will use ide-backend. For that use case, it makes more sense to grab the data files from the project's directory than copying to some other place.

[mgsloan]

Ideally, when using ide-backend locally, it would be possible to use their own directory for both source files and data files. I've looked into this a bit and opened an issue about this. Feel free to have a swing at that, but if it doesn't easily work out, I wouldn't prioritize it over the main yesod devel work.

Thanks for the attention to detail in your PR regarding commit messages and comments! I've got the following feedback:

• How about instead of a Bool, have a Maybe FilePath which specifies an override of the data file path? Then we can leave the session dir where it is, but use data files from a custom directory.
• I'd prefer to either leave ideSessionDataDir as it is or remove it - setting it to id seems wrong, as that means the data files will just go into the session directory. In particular, this means that updating a data file in the "src/" or "dist/" directories would overwrite that state.
• The "updateDataFile*" updates / getDataFile functions should either still work, or should throw exceptions. I may be reading the PR wrong, but I think if dataDirAsConfigDir is set to True, then these functions won't work / won't have an effect.

So, to get this merge, I'd either like to see my comments above addressed or an implementation of #280