Using Options to pick from multiple potential values

After getting the user's input, we need to
figure out a filename to write the file out
as. We can either use the title the user
passed in, or we can try to use heuristics
to find a title from the markdown content.

We can combine the user's title from the cli
arguments (an `Option`) with the logic we
use as a heuristic for the markdown (also
an `Option`).

```rust
// use `title` if the user passed it in,
// otherwise try to find a heading in the markdown
let document_title = title.or_else(|| {
    contents
        .lines()
        .find(|v| v.starts_with("# "))
        // markdown headings are required to have `# ` with
        // at least one space
        .map(|maybe_line| maybe_line.trim_start_matches("# ").to_string())
});
```

`option.or_else()` returns the original
option if it is a `Some` value, or executes
the function to return a different `Option`
if not. So `document_title` is an
`Option<String>` either way and we've
covered all of the possible scenarios:

1. the user has passed in a title
2. the user has written content with a heading
3. the user has done neither of these

Which leaves us to our markdown heuristic.

Markdown headings are required to have `# `
with at least one space, so we can turn
`contents` into an iterator and `find` the
first line that starts with `# `.

If we find one, we want to trim `# ` off of
the heading to get just the heading content,
so we can use `map` to operate on the value
inside of the `Option` returned by `find` if
it exists.

```rust
contents
    .lines()
    .find(|v| v.starts_with("# "))
    .map(|maybe_line| maybe_line.trim_start_matches("# ").to_string())
```

Now this code works fine, but it exposes a bug
in our file handling.

If we `write_all` to the `file`, that moves the
internal cursor to the end of that content. So
when we run our code now, `contents` is
skipping the first `# ` bytes, which means our
heuristic will only find the second heading in
the file.

To fix this, we can bring in the `std::io::Seek`
trait, and seek to the beginning of the file.

```rust
let mut contents = String::new();
file.seek(SeekFrom::Start(0))?;
file.read_to_string(&mut contents)?;
```

src/write.rs

@@ -1,6 +1,6 @@
 use color_eyre::{eyre::WrapErr, Result};
 use edit::{edit_file, Builder};
-use std::io::{Read, Write};
+use std::io::{Read, Seek, SeekFrom, Write};
 use std::path::PathBuf;
 
 const TEMPLATE: &[u8; 2] = b"# ";
@@ -19,8 +19,21 @@ pub fn write(garden_path: PathBuf, title: Option<String>) -> Result<()> {
     edit_file(filepath)?;
     // Read the user's changes back from the file into a string
     let mut contents = String::new();
+    file.seek(SeekFrom::Start(0))?;
     file.read_to_string(&mut contents)?;
 
-    dbg!(contents, title);
+    // use `title` if the user passed it in,
+    // otherwise try to find a heading in the markdown
+    let document_title = title.or_else(|| {
+        dbg!("here");
+        contents
+            .lines()
+            .find(|v| dbg!(v).starts_with("# "))
+            // markdown headings are required to have `# ` with
+            // at least one space
+            .map(|maybe_line| maybe_line.trim_start_matches("# ").to_string())
+    });
+
+    dbg!(contents, document_title);
     todo!()
 }