Closes #773

* zola init works without argument (defaults to current directory)

* zola init works with existing folders (no non-hidden files permitted)

* Update zola init documentation

* [docs] Optional argument is on a separate line for clarity

* Add tests for init subcommand's respect of existing folders

Closes #775

Closes #766

Closes #745

* Add hard_link_static config option.

* Copy or hardlink file depending on an argument.

Modify the call sites for `copy_file` to account for the extra argument.

* Plug the config setting through to copy_file.

Don't apply the config option to theme's static directory.

* Update documentation.

* Backticks make no sense in this comment.

* Addressing PR comments.

* Be consistent with argument naming.

* Add --open flag to serve command, to open URL in default browser

* Mention serve --open flag in CLI documentation

Closes #727

* Update cli-usage.md

* clarify that it's only the links from markdown files that are being checked;

It looks like two links on this page didn't get updated with the change from `./` to `@/` in ec61a57.

Close #686

* Add check subcommand

* Add some brief documentation for the check subcommand

* Start working on parallel link checks

* Check all external links in Site

* Return *all* dead links in site

Justification for this feature is added in the docs.

Precedent for the precise syntax: Hugo.

Hugo puts this syntax behind a preference named headerIds, and automatic
header ID generation behind a preference named autoHeaderIds, with both
enabled by default. I have not implemented a switch to disable this.

My suggestion for a workaround for the improbable case of desiring a
literal “{#…}” at the end of a header is to replace `}` with `}`.

The algorithm I have used is not identical to [that
which Hugo uses][0], because Hugo’s looks to work at the source level,
whereas here we work at the pulldown-cmark event level, which is
generally more sane, but potentially limiting for extremely esoteric
IDs.

Practical differences in implementation from Hugo (based purely on
reading [blackfriday’s implementation][0], not actually trying it):

- I believe Hugo would treat `# Foo {#*bar*}` as a heading with text
  “Foo” and ID `*bar*`, since it is working at the source level; whereas
  this code turns it into a heading with HTML `Foo {#<em>bar</em>}`, as
  it works at the pulldown-cmark event level and doesn’t go out of its
  way to make that work (I’m not familiar with pulldown-cmark, but I get
  the impression that you could make it work Hugo’s way on this point).
  The difference should be negligible: only *very* esoteric hashes would
  include magic Markdown characters.

- Hugo will automatically generate an ID for `{#}`, whereas what I’ve
  coded here will yield a blank ID instead (which feels more correct to
  me—`None` versus `Some("")`, and all that).

In practice the results should be identical.

Fixes #433.

[0]: a477dd1646/block.go (L218-L234)

Added and clarified a few things that would probably have saved me from
raising getzola/zola#691.

* bump version in deployment docs

Close #586

This reverts commit 1815155c1d.

If not specified, Travis use a system with ruby, which we don't need. We can use the "minimal" one instead.

Closes #530

Closes #636

Fix #613

Fix #616. Add brief documentation for how to use it.