bst source show

[Frazer Clews]

This is a proposal for `bst source show`

The idea is to show the sources in a BuildStream project
I think a normal `bst source show` should display the sources, along 
with their directories/URLs

`bst source show`

```

local: <example/dir>

git: <git_url>

https: <https_url>

```

Further goals would be to have certain flags to show only the 
directory/URL or the protocols in use

`bst source show -p|--protocol`?

```

local

git

https

```


`bst source show -u|--url`?

```

<example/dir>

<git_url>

<https_url>

```

Or one that only show a certain protocol, for example only git sources

`bst source show -g|--git`

```

<git_url>

```

Finally and not sure if this should be the default or one with flags, 
but one that shows the file names as headings, similar to `git grep 
--break --heading`

`bst source show -f|--file-name`?

```

hello.bst

local: <example/dir>

git: <git_url>

https: <https_url>

```

Open to suggestions/improvements to start work on `bst source show`, 
could also use arguements from `bst source fetch` which work well during 
testing

Thanks

Frazer