fix: fall back to original indentation keypress functionality

[mcauley-penney]

Problem: the press() function is used to fall back to the original functionality of <tab> and <S-tab> when not in a list context. When it does this, it appends before executing the original keypress, modifying the fall back functionality.

Solution: Do not append before using the fall back functionality of <tab> and <S-tab>.

This commit also refactors handle_indent(), - removes unused variables - removes run_recalculate_after_delay(), which appears to have no effect - reuses variables when possible (cur_line) - renames variables for brevity (in_list and at_line_end)

Fixes #79

[mcauley-penney]

@gaoDean I'd like a test run of this PR to see how it works for you

[pipoprods]

Just installed this modified version. I'll (hopefully) post feedback here.