Fix encoding error on Windows when not outputting to terminal

[ryanbuck182]

Closes #136

On Windows, when piping the command output somewhere else, it would cause an encoding error where the tree lines would be encoded as undesired special characters. The built-in tree command on Windows gets around this by automatically switching to an ASCII version of the tree. This PR makes a similar change to tre.

Now, on Windows, the output of something like tre | clip will be something like:

\-- example
    |-- Cargo.lock
    |-- Cargo.toml
    \-- src
        \-- main.rs

Instead of:

└── example
    ├── Cargo.lock
    ├── Cargo.toml
    └── src
        └── main.rs

This successfully avoids the encoding errors.

Full List of Changes:

• src/diagram_formatting.rs
  • added Charset enum for different sets of characters to render the diagram with (currently just Unicode and ASCII)
  • added PrefixSegment::as_str() method to return the appropriate string based on the Charset
  • make_prefix, format_file, and format_paths now accept a Charset parameter
  • updated formatting_works test to work with new function signatures
  • added formatting_ascii unit test for formatting with ASCII
  • extracted some shared code in both tests to a test_input() helper
• src/tre.rs
  • added check in run function that switches the Charset if on Windows and not outputting to a TTY
• tests/integration_tests.rs
  • added has_entry() helper so the tests check for the correct output characters (ASCII on Windows, Unicode otherwise)