Bash Compatibility: `+=` Operator

`conch-parser` incorrectly parses the += operator.

In the context of array concatenation:

```bash
  arr+=(appended)
# ^^^^^ ^^^^^^^^ -- Subshell Command
# | -- Command
```

In the context of string concatenation:

```bash
  foo+="bar"
# ^^^^^^^^^^ -- Command: Concat([Simple(Literal("foo+=")), DoubleQuoted([Literal("bar")])])