v0.29.0

Most relevant commits:

• compiler: always allow extended syntax in templates
• compiler/parser: make ParseSource function unexported
• compiler: split CompileProgram func in CompileProgram and CompileScript
• all: prefer 'script' to 'package-less program'
• compiler: use extended syntax in scripts
• all: prefer 'global' to 'builtin' for not Go builtins
• scriggo: add 'scripts' package and move code from the 'scriggo' package
• script: pass variable values as first argument to the Run function
• all: prefer 'packages' to 'loaders' and 'importers' for PackageLoader
• compiler/parser: fix panic parsing invalid token after template switch
• compiler/emitter: fix emitting of extending files with declarations
• compiler/parser: fix error message for case not in switch or select
• compiler/parser: fix panic importing and then showing the same file
• compiler/parser: check cycles before the already parsed check
• compiler/parser: fix 'extends' statement preceded by not empty text
• compiler/parser: fix panic showing an invalid path
• compiler/parser: add {% show(expr) %} statement
• compiler/checker: fix checking of partial files