add --only-latest flag

[tomoyamachi]

Currently, dockertags fetches all tag pages for summarize tags if set limit option.

+--------------------------------+------+----------------------+-------------+
|              TAG               | SIZE |      CREATED AT      | UPLOADED AT |
+--------------------------------+------+----------------------+-------------+
| 3                              | 2.7M | 2019-12-24T20:40:57Z | NULL        |
| 3.11                           |      |                      |             |
| latest                         |      |                      |             |
| 3.11.2                         |      |                      |             |
+--------------------------------+------+----------------------+-------------+
| edge                           | 2.7M | 2019-12-20T00:41:30Z | NULL        |
| 20191219                       |      |                      |             |
+--------------------------------+------+----------------------+-------------+
| 3.11.0                         | 2.7M | 2019-12-20T00:41:21Z | NULL        |
+--------------------------------+------+----------------------+-------------+
| 20191114                       | 2.7M | 2019-11-14T22:41:11Z | NULL        |
+--------------------------------+------+----------------------+-------------+
| 3.10                           | 2.7M | 2019-10-21T18:41:18Z | NULL        |
| 3.10.3                         |      |                      |             |
+--------------------------------+------+----------------------+-------------+

But It's enough to know first tag alias when CI pipeline.

$ dockertags --only-latest  -contains 3.10 alpine
+--------------------------------+------+----------------------+-------------+
|              TAG               | SIZE |      CREATED AT      | UPLOADED AT |
+--------------------------------+------+----------------------+-------------+
| 3.10                           | 2.7M | 2019-10-21T18:41:18Z | NULL        |
+--------------------------------+------+----------------------+-------------+