Fix recording of unicode characters in record (#64)

Fixes #63. (as in, I no longer see the error)

The fix is to replace

```julia
    i = min(length(rec.text), rec.cursor_index)
    a = rec.text[begin:i]
    b = rec.text[i+1:end]
```
with
```julia
    i = min(length(rec.text), rec.cursor_index)
    a = ""
    b = ""
    if i > 0
        a = rec.text[begin:nextind(rec.text, i-1)]
        b = rec.text[nextind(rec.text, i):end]
    end
```
which accounts for strings which are not uniformly indexed, like "aαbβcγ" which has its indices at `1, 2, 4, 5, 7, 8`.

src/changes.jl

@@ -13,8 +13,12 @@ Base.:(==)(x::BufferRecord, y::BufferRecord) =
 
 function Base.show(io::IO, rec::BufferRecord)
     i = min(length(rec.text), rec.cursor_index)
-    a = rec.text[begin:i]
-    b = rec.text[i+1:end]
+    a = ""
+    b = ""
+    if i > 0
+        a = rec.text[begin:nextind(rec.text, i-1)]
+        b = rec.text[nextind(rec.text, i):end]
+    end
     print(io, "BufferRecord(\"" * a * "|" * b * "\"), $(rec.cursor_index))")
 end