To use with Pythonista

indentation.py

import editor, console

script = editor.get_text()
selection = editor.get_selection()
selected_text = script[selection[0]:selection[1]].splitlines()
indentation = int(console.input_alert('Indent'))
replacement = []

for line in selected_text:
    if indentation > 0:
        replacement.append(('\t'*indentation)+line+('\n' if line != selected_text[-1] else ''))
    elif indentation == 0:
        replacement.append(line+('\n' if line != selected_text[-1] else ''))
    elif indentation < 0:
        if len(line) == 0:
            replacement.append('\n')
        elif line[0] == '\t':
            indent = min(abs(indentation), line.count('\t'))
            replacement.append(line.replace('\t', '', indent)+('\n' if line != selected_text[-1] else ''))
        else:
            replacement.append(line+('\n' if line != selected_text[-1] else ''))

editor.replace_text(selection[0], selection[1], ''.join(replacement))