Add script to sum changes status.

This script gives a summary of changes since the latest annotated tag.
Example output:

```
ivan in [/mnt/datos/Unixono/proyectos/LEAP]                                                                                                                      [45/9919]
(venv:bitmask) ➜ ./stats.sh 
Changes summary between closest annotated tag to HEAD
=====================================================

Stats for: bitmask_client
Stats: 116 files changed, 4316 insertions(+), 1377 deletions(-)
Merges: 559
----------------------------------------------------------------------
Stats for: leap_pycommon
Stats: 10 files changed, 263 insertions(+), 18 deletions(-)
Merges: 34
----------------------------------------------------------------------
Stats for: soledad
Stats: 84 files changed, 5104 insertions(+), 1375 deletions(-)
Merges: 216
----------------------------------------------------------------------
Stats for: keymanager
Stats: 12 files changed, 104 insertions(+), 66 deletions(-)
Merges: 76
----------------------------------------------------------------------
Stats for: leap_mail
Stats: 71 files changed, 9478 insertions(+), 2156 deletions(-)
Merges: 412
----------------------------------------------------------------------

TOTAL
Stats: 293 files changed, 19265 insertions(+), 4992 deletions(-)
Merges: 1297
```