Add total column for md and default

[gitsomebit]

closes #27

This pr does the following:

• adds a total column at the end of the table for md and default output.
• simplify code for printing seperator lines
• fix spacing of files column

I tested on the gocloc repo with the following results:

default

markdown

---

Additional comments:

• adding the total column made me realize that gocloc does not count all lines, with the total value being 1 less than the length of files. I am not sure if this is intended, so I wanted to mention it
• xml and json was not implemented, but wanted to get this out as a progressive enhancement
• when using gocloc on the gocloc repo I realized that for some reason gocloc by default excludes some folders. this seems to be the case for the .github folder as seen in the following output:

[gitsomebit]

@hhatto is something blocking this? would like to have this merged before opening another pr

[gitsomebit]

I would like to open a PR for custom OutputSeparators, but it would require this PR to be merged first