[Proposal] Incremental regeneration

[yosymfony]

Incremental regeneration helps shorten build times by only generating pages that were updated since the previous build. This first approach will be very simple and only regenerate a page if this one is changed.

Incremental regeneration can be enabled via the --incremental option in site:build command or by setting incremental: true in the configuration file of a site.

CLI examples:

spress site:build --incremental
spress site:build --server --incremental
spress site:build --server --watch --incremental

There will be a file called .metadata at the site root for keeping track of both file modification times and inter-document dependencies.

Limitations

• The config.yml file is excluded on this mode.
• The generators are excluded on this mode.

Tasks

• Save and read site metadata from a file called spress.metadata placed at site root.
• Manage the dependencies of an item (page) only considering the layout used by it.
• Modify the class FilesystemDataSource to recover only the modified files when the incremental mode is on.

Deal with: #97.