First implementation of html tag support.
Added customize option to specify "html" modes (where tags are supported).
sp-beginning-of-sexp now return the expression navigated on
`sp--unwrap-sexp` now removes the empty lines if the delimiters were the only thing on them. `sp-convolute-sexp` now respect the "lone" delimiters (nothing but a delimiter on a line).
Fixed `sp-get-thing` returning the tag epression even when the point was at a regular sexp.
Implemented the prefix-selectors. `sp-get-sexp` now correctly grabs the most "forward" expression (tag or paired)
Merge branch 'tag-support'