sel: enable multiple selectors

go get -u

sel: enable groups

sel: enable pseudo-elements

Improve multi error handling

Drop golint

go get -u

Add root filter

Fix send on closed channel

Further deadlock fixes

Send input datum from a separate goroutine

Fixes some deadlocks

Drop unused Expr.String()

s/git.sr.ht/d2evs.net/

Add license

Add totext filter

Add join filter

Send End datum at the end of the pipeline

Unused by current filters, but will be useful for filters which need to
do something when a channel is closed.

Expr.String: add parentheses for unambiguity

Add multi commands and parentheses

`f1 & f2 > f3` splits the input and passes it to f1 and f2, giving f3
both outputs as input. Parentheses are used to override precedence.

Add URL filters