Initial support for terminal within micro

This commit adds beta support for running a shell or other program
within a micro view.

Use the `> term` command. With no arguments, `term` will open your
shell in interactive mode. You can also run an arbitrary command
with `> term cmd` and the command with be executed and output
shown. One issue at the moment is the terminal window will close
immediately after the process dies.

No mouse events are sent to programs running within micro.

Ref #243

cmd/micro/lineArray.go

@@ -29,6 +29,8 @@ func runeToByteIndex(n int, txt []byte) int {
 	return count
 }
 
+// A Line contains the data in bytes as well as a highlight state, match
+// and a flag for whether the highlighting needs to be updated
 type Line struct {
 	data []byte
 
@@ -43,10 +45,12 @@ type LineArray struct {
 	lines []Line
 }
 
+// Append efficiently appends lines together
+// It allocates an additional 10000 lines if the original estimate
+// is incorrect
 func Append(slice []Line, data ...Line) []Line {
 	l := len(slice)
 	if l+len(data) > cap(slice) { // reallocate
-		// Allocate double what's needed, for future growth.
 		newSlice := make([]Line, (l+len(data))+10000)
 		// The copy function is predeclared and works for any slice type.
 		copy(newSlice, slice)
@@ -243,18 +247,22 @@ func (la *LineArray) Substr(start, end Loc) string {
 	return str
 }
 
+// State gets the highlight state for the given line number
 func (la *LineArray) State(lineN int) highlight.State {
 	return la.lines[lineN].state
 }
 
+// SetState sets the highlight state at the given line number
 func (la *LineArray) SetState(lineN int, s highlight.State) {
 	la.lines[lineN].state = s
 }
 
+// SetMatch sets the match at the given line number
 func (la *LineArray) SetMatch(lineN int, m highlight.LineMatch) {
 	la.lines[lineN].match = m
 }
 
+// Match retrieves the match for the given line number
 func (la *LineArray) Match(lineN int) highlight.LineMatch {
 	return la.lines[lineN].match
 }