Support wildcards in tag option

[junegunn]

Close #427

• No manual parsing of the expression; just uses git tag --list <pattern> for wildcard expansion
• ProTip: { 'tag: '*' } will point to the latest tagged release
• Requires git 1.9.2 or above (error: unknown option 'sort')
  • Are we fine with this? I think it's okay, as it's probably not going to be the most popular option.
  • So the test case fails on travis-ci as it comes with git 1.8.5.6.
    • We can either install the latest git using ppa, or just skip the test if the version requirement is not met
    • Since I'd like to make sure that vim-plug is still functional with older versions of git and Vim, so let's go with the latter.

/cc @vilhalmer

[vilhalmer]

Excellent, thanks so much!