v0.50.0

Most relevant commits:

• builtin: add 'pow' builtin
• cmd/scriggo: update help
• cmd/scriggo: fix documentation printed by 'scriggo help import'
• all: add the README
• ast, compiler: refer to 'extends' and 'import' as declarations
• compiler: make {% import "path" %} an alias for {% import . "path" %}
• all: stop execution on context cancellation
• playground: prefix the error message with "panic: " on panic
• runtime: change 'Zero' instruction to implement the new specification
• compiler/checker: fix panic in case of redeclaration of imported name
• all: update the copyright statements
• cmd: add 'pow' built-in to the 'serve' command
• runtime: fail with a fatal error on go of nil func value
• compiler/checker: don't export names imported in file/package block
• compiler/parser: fix parse composite literal with no type as expression
• compiler/lexer: fix lexing closed braces in show statement
• compiler: support recursive extends