Handle non-breaking space in markdown

[dannyp-aviva]

In the latest version of xc (0.8.6), including a non-breaking space (unicode 0xA0) within a markdown heading can cause a parsing error in xc. It seems to always report the parsing error in the task prior to the non-breaking space.

Example README.md:

## Tasks

### get

Install

```shell
go get ./...
```

### get

Run tests (note: there is a non-breaking space between `###` and `get` above)

```shell
go test ./...
```

Running xc gives this error:

xc parse error: command block already exists for task get