self-upgrade should prompt when no option was given

[tyru]

Current behavior

No prompt before upgrading.

$ volt self-upgrade
[INFO] Found update: v0.3.6 -> v0.3.7
---
* [Improvement] Sort array elements in lock.json (for diff output) (issue #255, PR #287, #284 @hupfdule)
* [Improvement] Support go modules (PR #283 @hupfdule)
* [Fix] Avoid duplicated repositories with different cases (issue #277 @hupfdule, PR #288)
* [Fix] "volt edit {repository}" drops {repository} (issue #280 @mrymtsk, PR #281 @hupfdule)
* [Fix] 'volt migrate' does not show help message (issue #286, PR #289)
* [Fix][Doc] Add more descriptions in README (issue #291, #272 @hupfdule, #273 @hupfdule, PR #292)
* [Fix] Re-implement broken transaction by empty lock directory (issue #290, PR #296)
* [Fix] Test only the nearest two major go versions (PR #293)
* [Fix] Cleanup tmp dirs after test (PR #285 @hupfdule)
---

Expected behavior

$ volt self-upgrade
[INFO] Found update: 0.3.6 -> v0.3.7
---
* [Improvement] Sort array elements in lock.json (for diff output) (issue #255, PR #287, #284 @hupfdule)
* [Improvement] Support go modules (PR #283 @hupfdule)
* [Fix] Avoid duplicated repositories with different cases (issue #277 @hupfdule, PR #288)
* [Fix] "volt edit {repository}" drops {repository} (issue #280 @mrymtsk, PR #281 @hupfdule)
* [Fix] 'volt migrate' does not show help message (issue #286, PR #289)
* [Fix][Doc] Add more descriptions in README (issue #291, #272 @hupfdule, #273 @hupfdule, PR #292)
* [Fix] Re-implement broken transaction by empty lock directory (issue #290, PR #296)
* [Fix] Test only the nearest two major go versions (PR #293)
* [Fix] Cleanup tmp dirs after test (PR #285 @hupfdule)
---

Upgrade? [y/N]: 

Add -yes option to skip prompt.

$ volt self-upgrade -yes
(same as before (no prompt))