Version 2.1.0-beta.1

```
* [New] Add `assets_dir`, `assets_url` and `plugins_url` config params
* [New] Add `%config.*%` Markdown placeholders for scalar config params and the
        `%assets_url%`, `%themes_url%` and `%plugins_url%` placeholders
* [New] Add `content-sample/theme.md` for theme testing purposes
* [New] Introduce API versioning for themes and support theme-specific configs
        using the new `pico-theme.yml` in a theme's directory; `pico-theme.yml`
        allows a theme to influence Pico's Twig config, to register known meta
        headers and to provide defaults for theme config params
* [New] Add `assets_url`, `themes_url` and `plugins_url` Twig variables
* [New] Add `pages` Twig function to deal with Pico's page tree; this function
        replaces the raw usage of Pico's `pages` array in themes
* [New] Add `url` Twig filter to replace URL placeholders (e.g. `%base_url%`)
        in strings using the new `Pico::substituteUrl()` method
* [New] Add `onThemeLoading` and `onThemeLoaded` events
* [New] Add `debug` config param and the `Pico::isDebugModeEnabled()` method,
        cehcking the `PICO_DEBUG` environment variable, to enable debugging
* [New] Add new `Pico::getNormalizedPath()` method to normalize a path; this
        method should be used to prevent content dir breakouts when dealing
        with paths provided by user input
* [New] Add new `Pico::getUrlFromPath()` method to guess a URL from a file path
* [New] Add new `Pico::getAbsoluteUrl()` method to make a relative URL absolute
* [New] picocms#505: Create pre-built `.zip` release archives
* [Fixed] picocms#461: Proberly handle content files with a UTF-8 BOM
* [Changed] Introduce API version 3
* [Changed] Rename `theme_url` config param to `themes_url`; the `theme_url`
            Twig variable and Markdown placeholder are kept unchanged
* [Changed] Update to Parsedown Extra 0.8 and Parsedown 1.8 (both still beta)
* [Changed] Enable Twig's `autoescape` feature by default; outputting a
            variable now causes Twig to escape HTML markup; Pico's `content`
            variable is a notable exception, as it is marked as being HTML safe
* [Changed] Rename `prev_page` Twig variable to `previous_page`
* [Changed] Mark `markdown` and `content` Twig filters as being HTML safe
* [Changed] Add `$singleLine` param to `markdown` Twig filter as well as the
            `Pico::parseFileContent()` method to parse just a single line of
            Markdown input
* [Changed] Add `AbstractPicoPlugin::configEnabled()` method to check whether
            a plugin should be enabled or disabled based on Pico's config
* [Changed] Deprecate the use of `AbstractPicoPlugin::__call()`, use
            `PicoPluginInterface::getPico()` instead
* [Changed] Update to Twig 1.36 as last version supporting PHP 5.3, use a
            Composer-based installation to use a newer Twig version
* [Changed] Add `$basePath` and `$endSlash` params to `Pico::getAbsolutePath()`
* [Changed] Deprecate `Pico::getBaseThemeUrl()`
* [Changed] Replace various `file_exists` calls with proper `is_file` calls
* [Changed] Refactor release & build system
* [Changed] Improve PHP class docs
* [Changed] Various small improvements
* [Removed] Remove superfluous `base_dir` and `theme_dir` Twig variables
* [Removed] Remove `PicoPluginInterface::__construct()`
```