A few small fixes / extra checks

[gkreitz]

A small PR gathering up a few fixes/improvements:

• Fixes a crash if one did load/check twice (and adds a test case)
• Adds error if there are problem names configured without matching statements
• Adds a function to check if validation uses the default validator (and simplify a bit of code using it)
• Adds a warning/error if there are multiple output validators
• Adds support for -i in md2html (setting a base URL for all image paths in the html)
• Fix dockerfile for full missing a lot of languages