Follow named anchors with ge

This commit allows ge to follow named anchors in links of the form
file#anchor, where file may omit the .md extension as usual.

Two global variables control the operation of this feature:

    vim_markdown_follow_anchor: a flag that tells vim-markdown whether
    to attempt to follow a named anchor in a link or not. When this is
    1, and only if a link can be split in exactly two parts by the
    pattern '#', then the first part is interpreted as the file and the
    second one as the named anchor. After the file is opened, the anchor
    will be searched.

    vim_markdown_anchorexpr: if available this expression will be
    evaluated substituting v:anchor with a quoted string that contains
    the anchor to visit. The result of the evaluation will become the
    real anchor to search in the target file. This is useful in order to
    convert anchors of the form my-section-title to searches of the form
    My Section Title. It needs to be generic since every markdown
    renderer follows its own conventions to generate "auto-anchors".
    A dummy example: let vim_markdown_anchorexpr = "'<<'.v:anchor"

Note: I've also reformatted EditUrlUnderCursor a bit since it was
violating the 4-space tab and single-quoted string conventions that most
of the rest of the code seems to follow.