UI improvements + slight refactoring of backend commands to support detection of proof mode.

Following SerAPI, 'Query' now handles requesting for goals and running
vernac queries (`Check`, `Print`).
It also handles 'Inspect', and a new query, 'Mode', is used to detect proof
mode. This removes the need for the previous hack where mode is returned
with Feedback(Processed) messages.

Coq standard library is now bundled in the `'coq'` package bundle.

Fixes #210.

`catch { }` causes syntax error in older Node.js?

Does not make sense to highlight vernac statements if hitting 'Ctrl'
is not going to have any effect.

When statements are reported as processed,
as part of the Feedback message.

Also, support for importing legacy `_CoqProject` definitions.

Because offsets returned from Coq are in bytes, whereas CodeMirror uses
chars.

This assumes that all strings sent to Coq are utf-8-encoded.

As long as all of a module's dependencies have compiled successfully.

(Also, some unrelated cleanup in file-view.)

Currently, only stores recently opened projects and files,
and only applies to physical directories (when running in NW.js/Electron).

[readme] mailing list -> Zulip

Thanks to enjoysmath, closes #206

Closes #204

They are no longer considered part of the statement
following them.
Going forward skips comments to get the next statement.

This allows for the use of Vue's template compiler at build time,
reducing package footprint.

Keyboard shortcuts are added via a CodeMirror.keyMaps entry.
In literate mode (snippets), PgUp/PgDn are not bound, to allow scrolling.

More features and cleanup of project pane functionality.

'Init' does not create the document anymore. The client calls both
when it starts.

For batch compilation, compile with `Stm.VoDoc` type.
Terminate worker after batch finishes up.

Introduced a new command, 'Start', for use in multi-file compilation.

Mostly, to allow building in physical directories
(in subprocess mode).

In fact there was a typo in some dead code.

Lib.init() resets the global env. So all the flags etc. must be set after it.

Fixes #201.

[travis] Don't use incorrect git submodule call

This is updating not to the pinned module but to the remote, which
will make CI fail.

Travis should take care of the submodules already via

```
git:
  submodules: true
```

which IIANM is the default.

^^