Attachment replacements in link texts

[IMFJS]

As described in the readme, attachment paths should avoid being subsets of the contents of the Markdown file, because Mark currently just performs a simple ReplaceAll operation where it replaces any occurrence of an attachment path with the Confluence URL to that attachment. However, this is unfortunate in cases like [Attachment.txt](Attachment.txt) where the attachment is located next to the Markdown file and the link text has the same name (and extension) as the attachment in order to provide a download link for this file. Of course there are workarounds like putting the attachments into a subfolder like "assets" or slightly altering the link text, but still, this is a big limitation, especially for providing downloads in Confluence.

I already wrote a fix for this specific case and will shortly open a pull request.

[IMFJS]

So I have found a better solution to this issue than I first had in mind which is to simply refer to the attachment using the current-directory specifier ./ as follows:
Instead of

<!-- Attachment: Sample.txt -->
[Sample.txt](Sample.txt) becomes [/download/xxx](/download/xxx)

do

<!-- Attachment: ./Sample.txt -->
[Sample.txt](./Sample.txt) becomes [Sample.txt](/download/xxx)

However, this workaround/solution requires #185 to be merged, because otherwise the filename in Confluence would change from Sample.txt to ._Sample.txt.