v0.1.7

v0.1.7: String operations and file.read command

New features:
- file.read: Read file content into variable or print
- string.replace: Replace pattern in text
- string.contains: Check if string contains pattern
- string.length: Get string length
- string.upper: Convert to uppercase
- string.lower: Convert to lowercase
- string.trim: Remove whitespace

All string operations support var= parameter to store result in variable.
Updated parser to recognize string.* family.
Updated help system with new commands.