build ramda.js from multiple source files

Compare the output of the following commands:

```command
$ ./scripts/build -- lib/head.js
```

```command
$ ./scripts/build -- lib/head.js lib/tail.js
```

```command
$ ./scripts/build --complete
```

__ramda.js__ is built by running the last of these commands. If the `VERSION` environment variable is set (by xyz, for example) its value is used in the version string in the header (see #617).

Modular builds have been discussed in #577 and elsewhere. This pull request gives us the ability to create extremely granular custom builds. Only the requested functions are exported, and functions not required by any of the requested functions or any of their dependencies are not included at all.

There is a reason, though, that I didn't entitle this pull request "support custom builds". We don't expect users to create custom builds by specifying hundreds of filenames. We'd like to support something like this:

```console
$ ./scripts/build --category logic --category math
```

That change should be fairly easy to implement, but I'd like it to be reviewed and discussed in a separate pull request once this has been merged. This pull request is already large enough!

I'll add further comments in situ.

I guess we never got there with Ramdocs.  That was supposed to be the goal.  I thought that someone had already done that part.  

I know that there are tools to help with this.  But one thing not to worry about would be merge conflicts.  This would be a generated file, that **only** Travis should be checking in.  It can use brute force for that.

My concern with the current process is that we now have to monitor it.  Did the change touch the core files?  If so, did the user rebuild Ramda?  Should we be double-checking that they actually rebuilt since their last change?

I suppose any inconsistencies creeping in with this would only last until the next person went to commit, doing it the right way.  But it still feels wrong.  I would rather depend on tools than process if at all possible.