Change markup syntax and add underline/overline.

[ghost]

I think most people are used to using standard bbcode syntax. Also it messes up ``these quotes'' when done twice and italicizes text between it.

Also added overline/underline.

[czaks]

it depends. if people come from KusabaX, then it's true, that tey are used to BBCode.

if not, well. mediawiki, reddit, markdown (i.e. github), all use some sort of markup using quotes. there are also many tinyboard boards and merging this PR would break behaviour for the users.

all i can suggest is introduce two config variables. one enabled by default, with the old functionality and one disabled by default enabling bbcodes.