Added handling of absolute path to vimwiki

[justrajdeep]

• update vimwiki link handling so that if using env variables it is resolved correctly
• add [[//absolute_path]] to handle absolute path
• updated doc to reflect the changes
• Fix [Question] Markdown: point a file with absolute path #746

Steps for submitting a pull request:

• ALL pull requests should be made against the dev branch!
• Take a look at CONTRIBUTING.MD
• Reference any related issues.
• Provide a description of the proposed changes.
• PRs must pass Vint tests and add new Vader tests as applicable.
• Make sure to update the documentation in doc/vimwiki.txt if applicable,
  including the Changelog and Contributors sections.

[tinmarino]

Change test path to be docker coherent

[tinmarino]

This absolute path specific to your machine ?

[justrajdeep]

Yes. Sorry!! nice catch ... will remove it

[tinmarino]

Neat ! With doc and test. All in one commit. Seems clean.
I think the test won't work on my machine, the path is specific

[tinmarino]

It is not working on my machine.
I tried a markdown wiki with:

[test1](//home2/tourneboeuf/Test/Wiki/test.md)

I improved a little bit your Vader test. But itdoes not work as the absolute link is not creating so the filetype is not the one expected.

# TODO mutualize with link_creation

Execute:
  Log 'Hello !'

Given vimwiki(some wiki link):
  [[//home/vimwiki/testmarkdown/absolute_link1.md|absolute link 1]]
  [absolute link](//home/vimwiki/testmarkdown/absolute_link2.md)

Execute (check vimwiki#base#resolve_link functionality):
#  setf pandoc
#  AssertEqual vimwiki#base#resolve_link('/tmp/absolute_link')['filename'], '/tmp/absolute_link'

Do (Enter):
  :file! $HOME/testmarkdown/given.md\<Cr>
  :AssertEqual &ft, 'vimwiki'\<Cr>
  \<Tab>
  \<Tab>
  \<Cr>
  \<Cr>
  \<Cr>
  :AssertEqual @%, 'toto'\<Cr>

[tinmarino]

Hi,

Rebased manually: e3e841f
I add to do some modifications, and kept all in only one commit. I hope you don't feel stolen for that. I left your name and comment in the commit.

Tests are important: For example, the raw inclusion of your modifications broke some : https://travis-ci.com/github/vimwiki/vimwiki/builds/176612972
because let link_text = resolve(expand(link_text)) was breaking local links like [local](#heading)

Anyway good job, it is working.
See You.